V658 Carinae: Eclipsing Be+sdOB Binary

• V658 Carinae is an eclipsing interacting binary featuring a rapidly rotating Be star with a luminous equatorial decretion disk and a hot, low-mass stripped companion.
• Recent studies combined photometry, spectroscopy, polarization, and TESS data to derive precise orbital parameters, including a near edge-on inclination and a 32.185-day period.
• Modeling reveals a complex post-mass-transfer evolution with debates on stellar classification that impact our understanding of Be-star disk physics and chemically peculiar stars.

V658 Carinae, also designated HD 92406, is an eclipsing interacting binary whose interpretation has evolved from a post-Algol shell-star system to a benchmark eclipsing Be-star binary with a hot stripped companion. Across recent studies, the system is consistently described as nearly edge-on, with an orbital period near $32.185$ d, a luminous equatorial decretion disk, and a hot low-mass secondary that is not a normal main-sequence B star. Its importance derives from the unusual conjunction of two stellar eclipses and broad disk-related attenuation events, which allow the disk, the stripped remnant, and the aftermath of binary mass transfer to be studied in the same object (Hauck, 2018, Amorim et al., 26 Jul 2025, Hauck, 1 Feb 2026).

1. Identification and defining geometry

V658 Car has been presented as an almost edge-on eclipsing binary with mutually consistent but not identical orbital ephemerides in successive analyses. Hauck gave $P = 32.1854(1)$ d and epoch of primary minimum HJD $2452786.438(2)$, with inclination $i = 88.72^\circ\,(-0.09/+0.18)$ and a circular orbit assumed. The 2026 renewed study quoted $P = 32.18537(2)\,\mathrm{d}$, the same HJD epoch $2452786.438(2)$, inclination $88.75 \pm 0.10^\circ$, and a circular orbit. The 2025 SB2 analysis derived $P = 32.18534(2)\ \mathrm{d}$, $T_0 = 2459545.368(1)\ \mathrm{BJD}$, and $i = 88.6(1)^\circ$, with $P = 32.1854(1)$0, i.e. essentially circular (Hauck, 2018, Amorim et al., 26 Jul 2025, Hauck, 1 Feb 2026).

The light-curve morphology is the system’s defining observational feature. The 2025 study described two narrow stellar eclipses at phases 0 and 0.5, embedded in two broad, shallower attenuation events. The first attenuation is asymmetric, steep and abrupt, extends roughly from phase $P = 32.1854(1)$1 to $P = 32.1854(1)$2, and includes a local brightening near the first eclipse, producing a W-shaped structure. The second attenuation is broader, smoother, nearly symmetric, and extends roughly from phase $P = 32.1854(1)$3 to $P = 32.1854(1)$4. In the 2018 interpretation, the “outer primary eclipse” begins and ends at phases 0.905 and 0.115, with total central star eclipse between phases $P = 32.1854(1)$5 and $P = 32.1854(1)$6; the minimum duration is given as 12.4 hours (Hauck, 2018, Amorim et al., 26 Jul 2025).

This geometry led Hauck to emphasize two peculiarities: the shell star itself is eclipsed together with its decretion disk, and the eclipsing object is hot but only $P = 32.1854(1)$7 in the 2018 solution. The 2025 work reformulated the same phenomenology as the first confirmed eclipsing Be + sdOB system and the only known eclipsing Be + sdOB binary, in which the hot companion directly probes the Be-star disk (Hauck, 2018, Amorim et al., 26 Jul 2025).

2. Observational basis

The empirical basis for V658 Car combines photometry, spectroscopy, and, in the most recent detailed analysis, polarization. The 2018 study used continued photometry at Siding Spring Observatory with a remote 0.5 m reflector in $P = 32.1854(1)$8, and $P = 32.1854(1)$9, together with additional $2452786.438(2)$0 and $2452786.438(2)$1 photometry of central eclipses from ROAD observatory in San Pedro, Chile, using a 0.4 m reflector. The phase-folded light curves were based on 167 data points in $2452786.438(2)$2, 139 in $2452786.438(2)$3, 138 in $2452786.438(2)$4, and 104 in $2452786.438(2)$5. The ephemeris was tied to ASAS plus the new data, and the period was stated to have remained constant over the last 17 years (Hauck, 2018).

The same 2018 work reexamined historical radial velocities from F. Gieseking (1981). Hauck removed the first 3 points of the second data set, noting that points 1 and 3 had already been rejected by Gieseking, and reinterpreted 4 of the remaining 14 measurements as belonging to the bright low-mass secondary rather than the primary. The paper also used RAVE DR5 for a disk temperature estimate and BESS spectra for $2452786.438(2)$6 profile behavior (Hauck, 2018).

The 2025 study assembled a much broader data set. It used TESS photometry from 5 sectors, optical spectroscopy from NRES/LCO with 92 spectra over 380–860 nm at $2452786.438(2)$7, one older BeSS spectrum, ultraviolet spectroscopy from IUE, Gaia DR3 BP/RP spectra, near-infrared spectroscopy from SOAR/TripleSpec over 945–2465 nm at $2452786.438(2)$8, and OPD broad-band $2452786.438(2)$9 polarization. The combined observables formally fitted were the UV spectrum, optical spectrum, IR spectrum, TESS light curve, polarization, and $i = 88.72^\circ\,(-0.09/+0.18)$0 equivalent width, while absorption-line profiles were excluded because the required physics was not yet implemented (Amorim et al., 26 Jul 2025).

The 2026 renewed study used photometric $i = 88.72^\circ\,(-0.09/+0.18)$1 data from a remotely controlled CDK 20-inch telescope in Siding Spring, TESS light curves, and radial-velocity data replacing earlier incomplete measurements with the secondary-star semi-amplitude from de Amorim et al. (2025), $i = 88.72^\circ\,(-0.09/+0.18)$2. Orbital light-curve modeling was carried out with Binary Maker 3, with the geometry derived mainly from the best fit to the U-band primary minimum and an achieved fit residual $i = 88.72^\circ\,(-0.09/+0.18)$3 (Hauck, 1 Feb 2026).

3. Orbital and dynamical solutions

The dynamical solution proposed in 2018 differed substantially from later SB2-based modeling. Hauck obtained $i = 88.72^\circ\,(-0.09/+0.18)$4, $i = 88.72^\circ\,(-0.09/+0.18)$5, $i = 88.72^\circ\,(-0.09/+0.18)$6, $i = 88.72^\circ\,(-0.09/+0.18)$7, and $i = 88.72^\circ\,(-0.09/+0.18)$8. The reclassification of some historical RV points as secondary-star measurements improved OFIT from $i = 88.72^\circ\,(-0.09/+0.18)$9 to $P = 32.18537(2)\,\mathrm{d}$0 using Binary Maker 3. This amended RV interpretation was central to the lower-mass primary proposed in that paper (Hauck, 2018).

The 2025 work instead derived an SB2 orbital solution from low-excitation lines associated with the Be star and high-excitation lines associated with the companion. It reported $P = 32.18537(2)\,\mathrm{d}$1, $P = 32.18537(2)\,\mathrm{d}$2, $P = 32.18537(2)\,\mathrm{d}$3, and systemic velocity $P = 32.18537(2)\,\mathrm{d}$4. The orbital semimajor axes were $P = 32.18537(2)\,\mathrm{d}$5 and $P = 32.18537(2)\,\mathrm{d}$6, implying a total separation of about $P = 32.18537(2)\,\mathrm{d}$7. Eggleton-based Roche lobe radii were $P = 32.18537(2)\,\mathrm{d}$8 and $P = 32.18537(2)\,\mathrm{d}$9 (Amorim et al., 26 Jul 2025).

The 2026 renewed study adopted a closely similar large-scale orbital geometry, quoting semi-major axis $2452786.438(2)$0, stellar eclipse duration $2452786.438(2)$1, disk eclipse duration $2452786.438(2)$2, distance $2452786.438(2)$3, and extinction $2452786.438(2)$4. By contrast, the 2025 modeling gave $2452786.438(2)$5 and $2452786.438(2)$6. This literature divergence extends beyond the masses to the global photometric solution and line-of-sight extinction (Amorim et al., 26 Jul 2025, Hauck, 1 Feb 2026).

The 2026 paper explicitly referenced the standard Keplerian relation

$2452786.438(2)$7

and the standard circular-orbit mass function

$2452786.438(2)$8

stating that the resulting $2452786.438(2)$9 and $88.75 \pm 0.10^\circ$0 simultaneously comply with the mass function calculated with this $88.75 \pm 0.10^\circ$1 (Hauck, 1 Feb 2026).

4. Stellar components and disk

The stellar and disk parameters proposed for V658 Car differ markedly between studies. In Hauck’s 2018 amended interpretation, the primary was reassigned to spectral type A0p(e shell), with $88.75 \pm 0.10^\circ$2 K, $88.75 \pm 0.10^\circ$3, $88.75 \pm 0.10^\circ$4, $88.75 \pm 0.10^\circ$5, and $88.75 \pm 0.10^\circ$6. The secondary, observationally B-type but physically interpreted as a white-dwarf precursor, was given $88.75 \pm 0.10^\circ$7 K, $88.75 \pm 0.10^\circ$8, $88.75 \pm 0.10^\circ$9, $P = 32.18534(2)\ \mathrm{d}$0, and $P = 32.18534(2)\ \mathrm{d}$1. The disk had $P = 32.18534(2)\ \mathrm{d}$2 K and $P = 32.18534(2)\ \mathrm{d}$3. At maximum light, the relative contributions were given as $P = 32.18534(2)\ \mathrm{d}$4 0.173, 0.534, 0.293; $P = 32.18534(2)\ \mathrm{d}$5 0.202, 0.445, 0.353; $P = 32.18534(2)\ \mathrm{d}$6 0.183, 0.382, 0.435; $P = 32.18534(2)\ \mathrm{d}$7 0.142, 0.347, 0.511, for primary, secondary, and disk respectively (Hauck, 2018).

The 2025 SB2 and radiative-transfer analysis replaced this with a classical Be-star solution. The primary was modeled as an oblate rapidly rotating Be star with $P = 32.18534(2)\ \mathrm{d}$8, polar radius $P = 32.18534(2)\ \mathrm{d}$9, equatorial radius $T_0 = 2459545.368(1)\ \mathrm{BJD}$0, polar temperature $T_0 = 2459545.368(1)\ \mathrm{BJD}$1, luminosity $T_0 = 2459545.368(1)\ \mathrm{BJD}$2, and equatorial rotation speed $T_0 = 2459545.368(1)\ \mathrm{BJD}$3, with fixed $T_0 = 2459545.368(1)\ \mathrm{BJD}$4. The companion was modeled as essentially spherical and slow-rotating, with $T_0 = 2459545.368(1)\ \mathrm{BJD}$5, $T_0 = 2459545.368(1)\ \mathrm{BJD}$6, $T_0 = 2459545.368(1)\ \mathrm{BJD}$7, $T_0 = 2459545.368(1)\ \mathrm{BJD}$8, and $T_0 = 2459545.368(1)\ \mathrm{BJD}$9. The companion dominates the UV and contributes about 31% of the optical flux. The Be disk was assigned outer radius $i = 88.6(1)^\circ$0, base density $i = 88.6(1)^\circ$1, mean temperature about $i = 88.6(1)^\circ$2, and TESS-band optical depths up to $i = 88.6(1)^\circ$3 (Amorim et al., 26 Jul 2025).

The 2026 renewed study produced a third quantitative solution. It described the primary as a Be star with $i = 88.6(1)^\circ$4, mean radius $i = 88.6(1)^\circ$5, polar/equatorial radii $i = 88.6(1)^\circ$6, $i = 88.6(1)^\circ$7, $i = 88.6(1)^\circ$8, and $i = 88.6(1)^\circ$9. The secondary was assigned $P = 32.1854(1)$00, $P = 32.1854(1)$01, $P = 32.1854(1)$02, and $P = 32.1854(1)$03. At maximum light in $P = 32.1854(1)$04, the adopted flux fractions were 0.207 for the primary, 0.386 for the secondary, and 0.407 for the disk. The same paper gave multiple primary temperatures depending on geometry and disk dimming: $P = 32.1854(1)$05 in secondary minimum, $P = 32.1854(1)$06 at maximum light, $P = 32.1854(1)$07 for the Be star without disk in the line of sight, $P = 32.1854(1)$08 over its total surface area according to BM3 output for an equivalent sphere, and $P = 32.1854(1)$09 as the theoretical effective temperature of the corresponding non-rotating spherical star (Hauck, 1 Feb 2026).

These differences are not limited to parameter refinement; they encode different physical pictures of the primary. The 2018 paper preferred an A0p shell-star primary. The 2025 analysis argued instead for a classical Be star, rapidly rotating and gravity-darkened. The 2026 paper retained the Be-shell framework but added the claim that the star is chemically peculiar and observed through disk dimming (Hauck, 2018, Amorim et al., 26 Jul 2025, Hauck, 1 Feb 2026).

5. Evolutionary interpretation and disk physics

All three studies interpret V658 Car as the product of binary mass transfer. Hauck’s 2018 paper described the system as post-Algol: the current primary is the rejuvenated, spun-up accretor, while the current secondary is the stripped donor remnant. Using the evolutionary calculations of Istrate et al. (2014), Hauck stated that star 2 lies on the track of a proto-helium white dwarf of $P = 32.1854(1)$10 and has an age of not more than 7 Myr since Roche-lobe detachment. The paper also noted consistency with the white-dwarf remnant mass–final orbital period relation for stable Roche-lobe overflow from Carter et al. (2011) (Hauck, 2018).

The 2025 analysis kept the post-RLOF framework but reidentified the stripped component as sdOB, more specifically closer to sdB / early-B-like stripped companion than an O-type subdwarf. The Be star was interpreted as a late-type classical Be star spun up by past accretion, and the hot compact secondary as a stripped helium-rich remnant. The UV SED, where the companion is on average 3.5 times brighter than the Be star, was presented as a classic hallmark of stripped companions in Be binaries, while the optical/NIR SB2 nature made V658 unusual within that class (Amorim et al., 26 Jul 2025).

The decretion disk is central to every interpretation. Hauck’s 2018 study argued that photometry confirmed the concave shape of the large decretion disk surrounding the primary. The decisive evidence was the presence of additional minima at phases 0.965 and 0.035, interpreted as the secondary passing through the thicker outer regions of a concave disk, then through vertically thinner inner regions, before entering central star eclipse. This was presented as direct optical confirmation of the standard model of a concavely shaped decretion disk, as discussed by Rivinius et al. The same paper reported shell-star-typical $P = 32.1854(1)$11 emission profiles in BESS spectra, with central absorption increasing toward phases 0 and 0.5 (Hauck, 2018).

The 2025 work broadened the disk interpretation from simple concavity to non-axisymmetric binary-shaped structure. It modeled the first attenuation as Be-disk attenuation of the companion’s light and estimated the size of the attenuating region via

$P = 32.1854(1)$12

Applied to the first attenuation, this yielded $P = 32.1854(1)$13, matching the Be-star Roche lobe of $P = 32.1854(1)$14; applied to the second attenuation, it yielded $P = 32.1854(1)$15, too large for the companion Roche lobe. The paper therefore rejected a simple circumsecondary-disk explanation for the second attenuation. Additional spectroscopic evidence included weak double-peaked \ion{He}{1} emission co-moving with the companion, suggesting a tenuous circumsecondary envelope or disk-like structure, and a moving component in the NIR \ion{Fe}{2} 9997 Å line with $P = 32.1854(1)$16 and $P = 32.1854(1)$17, interpreted as likely arising in a bridge-like stream from the Be disk toward the companion (Amorim et al., 26 Jul 2025).

The 2026 renewed study assigned the system an even younger post-mass-transfer age, not more than about $P = 32.1854(1)$18 after the end of mass transfer, using the post-mass-transfer tracks of Iben & Tutukov with metallicity corrections from Istrate et al. It inferred an initial donor mass of approximately $P = 32.1854(1)$19 and stated that this leaves only $P = 32.1854(1)$20 for the initial accretor, implying a very extreme original mass ratio (Hauck, 1 Feb 2026).

6. Revisions, controversies, and astrophysical status

The history of V658 Car is primarily a history of reinterpretation. Hauck’s 2018 paper explicitly revised an earlier B5Vp-shell identification by reassigning the shell star to A0p, partly on the basis of Gieseking’s earlier classification and the observation that shell stars need not be B-type. That reinterpretation, together with the reassignment of some historical RV points to the low-mass secondary, produced the $P = 32.1854(1)$21 primary and $P = 32.1854(1)$22 white-dwarf-precursor solution (Hauck, 2018).

The 2025 study explicitly overturned that A0+B interpretation. It argued that V658 is not an eclipsing SB2 with an A0 primary plus circumstellar disk and a less massive B-type companion, but instead a rapidly rotating classical Be star orbited by a compact, hot, low-mass stripped companion. In this view, the system is the first confirmed eclipsing Be + sdOB system, the only known eclipsing Be + sdOB binary, and the second known late-type Be + stripped-star system after $P = 32.1854(1)$23 Dra. The same work also emphasized unresolved modeling failures: the second attenuation is not reproduced at all, the $P = 32.1854(1)$24 equivalent width behavior is wrong during the first attenuation, the NIR excess is overpredicted by about 30%, and the $P = 32.1854(1)$25- and especially $P = 32.1854(1)$26-band polarization are overpredicted. Those discrepancies were used to argue for future inclusion of companion radiative feedback and asymmetric disk structure informed by SPH (Amorim et al., 26 Jul 2025).

The 2026 renewed study accepted the post-mass-transfer Be-star framework but advanced a further claim: the Be star should be chemically peculiar. The reasoning was that Georgy et al. models place the primary close to the ZAMS only for sub-solar metallicity $P = 32.1854(1)$27, yet at that metallicity and mass the models imply $P = 32.1854(1)$28, about $P = 32.1854(1)$29 higher than the observationally based estimate; the abstract rounded this to about $P = 32.1854(1)$30. The paper therefore concluded that the primary should belong among the chemically peculiar stars despite its rapid rotation, and, citing Netopil et al., stated that it would then be one of the fastest rotators known among CP stars. TESS periods of $P = 32.1854(1)$31 and sometimes $P = 32.1854(1)$32 were interpreted through an oblique rotator model with a dipolar magnetic field whose axis is inclined to the rotation axis (Hauck, 1 Feb 2026).

A common misconception in the older literature is that the hot component can be treated as an ordinary B-type star because of its spectrum. All three studies reject that reading in different terms: white-dwarf precursor in 2018, stripped sdOB or early-B-like stripped companion in 2025, and contracting hot subdwarf precursor in 2026 (Hauck, 2018, Amorim et al., 26 Jul 2025, Hauck, 1 Feb 2026).

Another unresolved issue concerns the primary’s classification. The 2018 paper preferred A0p(e shell); the 2025 work preferred classical Be star; the 2026 work argued for a chemically peculiar Be-shell star in a sub-solar metallicity solution. This suggests that the system’s basic post-mass-transfer architecture is now more secure than the detailed atmospheric classification of the primary or the full three-dimensional structure of its circumstellar environment (Hauck, 2018, Amorim et al., 26 Jul 2025, Hauck, 1 Feb 2026).

7. Place in current research

Despite these disagreements, V658 Car is consistently treated as astrophysically exceptional. Hauck’s 2018 conclusion was that it offers a rare laboratory for rapidly rotating shell/decretion-disk stars, eclipse mapping of disk structure including evidence for a concave disk profile, and post-Algol evolution with the donor remnant observed during the brief proto-helium-white-dwarf contraction phase (Hauck, 2018).

The 2025 paper elevated that status to that of a benchmark system. Its combination of SB2 spectroscopy, eclipses, disk occultations or attenuations, polarization modulation, and UV-to-NIR SED coverage makes it unusually constraining for Be-star disk physics, stripped-star characterization, post-RLOF binary evolution, and binary-induced disk structure. The system’s unique geometry was summarized there as an astrophysical “X-ray” of a Be disk (Amorim et al., 26 Jul 2025).

The 2026 study added a further implication: if its interpretation is correct, V658 Car provides evidence that a rapidly rotating post-accretion Be star can remain chemically peculiar under the right gravity and magnetic-field conditions, even though rapid rotation is often expected to erase stable abundance patches through rotational mixing. That claim remains model-dependent, but it identifies V658 Car as a test case at the intersection of Be-star phenomenology, CP-star physics, and stripped-remnant evolution (Hauck, 1 Feb 2026).