NO Puppis: Young Hierarchical Eclipsing Binary

• NO Puppis is a young, early-type eclipsing binary in a hierarchical quadruple system marked by an eccentric orbit influenced by Kozai-Lidov cycles.
• Its absolute parameters were precisely determined using combined TESS photometry, ground-based BVR data, high-resolution spectroscopy, and Gaia astrometry.
• The system exhibits intriguing pulsational behavior and rotational dynamics that provide critical insights into tidal interactions and binary evolution.

Searching arXiv for NO Puppis and closely related papers to support the article. NO Puppis is the southern early-type, young, eccentric-orbit eclipsing binary identified as the A component of the multiple star Gaia DR3 552-8147999779517568. In the recent determination of its absolute parameters, the close binary consists of Aa and Ab, while the B component is an astrometric binary at a separation of about 8.1 arcsec; there may also be other fainter stars in the wider system. The current observational synthesis combines TESS photometry from four sectors, new ground-based BVR photometry, HARPS and HERCULES high-resolution spectroscopy, and astrometry, yielding a revised parameter set with increased precision and framing NO Puppis as a young hierarchical multiple whose short-period eccentric orbit, pulsational content, and rotational state are of particular interest (Erdem et al., 7 Aug 2025).

1. System definition and multiplicity

NO Puppis, also catalogued as HD 71487, is described as a young, early-type eclipsing binary whose close pair constitutes the “A” system, Aa+Ab. This subsystem is embedded in a more complex multiple-star configuration. The B component is an astrometric binary, Ba+Bb, now at a separation of about 8.1 arcsec from the A component, and the broader configuration may include additional faint stars (Erdem et al., 7 Aug 2025).

The structure inferred from the reported analysis is that of a likely hierarchical quadruple, expressed as $[(Aa,Ab)] + [(Ba,Bb)]$. The reported masses of the wide companions are approximate rather than fully dynamical: $M_{Ba} = 2.0$ and $M_{Bb} = 1.8$ in solar units. For the Ba,Bb orbit, the astrometric solution gives $a = 0.179 \pm 0.012$ arcsec, $P = 101.3 \pm 3.8$ yr, and $e = 0.054 \pm 0.041$, with an inclined orbit estimated from mass constraints but subject to large astrometric uncertainties (Erdem et al., 7 Aug 2025).

This multiplicity is central to the system’s interpretation. A plausible implication is that the dynamics of the wider components are not merely incidental, because the presence of Ba,Bb is explicitly discussed as enabling Kozai-Lidov forcing on the close binary. That connection becomes especially important in attempts to explain the persistence of non-zero eccentricity in the short-period A subsystem.

2. Observational basis and modelling framework

The published solution for NO Puppis is based on multiple observational channels. TESS observations from four sectors, designated S34, S35, S61, and S62, were obtained in short cadence at 120 s and span roughly 2021–2023. These light curves were analysed with both WinFitter and the Wilson-Devinney code coupled with Monte Carlo optimisation for parameter estimation and uncertainty evaluation. The TESS data yielded precise constraints on fractional stellar radii, orbital inclination, eccentricity, and third light (Erdem et al., 7 Aug 2025).

Ground-based multicolour BVR photometry obtained over six nights was used both independently and jointly with TESS data. The reported workflow includes deblending of light contributions from all components and their use in photometric parallax and SED fitting. Spectroscopy provided the complementary dynamical constraints: 34 HARPS high-S/N spectra acquired over a 5-day window were used for precise radial velocities, while additional UCMJO HERCULES echelle spectra supported rotational analysis through the He I $\lambda 6678$ line. RV analysis was performed with both Wilson-Devinney and WinFitter, and the orbital solution was constrained by rapid apsidal motion (Erdem et al., 7 Aug 2025).

Spectral disentangling was carried out with KOREL and FDBinary, and synthetic Kurucz spectra were fitted to derive temperatures, gravities, projected rotational velocities, and metallicities. In parallel, 53 times of minima compiled from historical data and new TESS/BVR measurements were fitted for orbital elements and apsidal motion, including linear and cyclic $O-C$ analysis. The astrometric orbit of the B subsystem was fitted with MCMC using WDS catalog data. Taken together, these procedures define a strongly overconstrained solution in which photometric, spectroscopic, and astrometric evidence are reported to be in strong mutual agreement (Erdem et al., 7 Aug 2025).

3. Absolute parameters of the close eclipsing binary

The revised absolute parameters of the close binary Aa+Ab are the most specific quantitative outcome of the study. The primary has $M_{Aa} = 3.58 \pm 0.11$, $R_{Aa} = 2.17 \pm 0.03$, and $M_{Ba} = 2.0$0, while the secondary has $M_{Ba} = 2.0$1, $M_{Ba} = 2.0$2, and $M_{Ba} = 2.0$3, in solar or kelvin units as appropriate. Additional reported parameters are $M_{Ba} = 2.0$4 cgs for Aa and $M_{Ba} = 2.0$5 cgs for Ab, luminosities of $M_{Ba} = 2.0$6 and $M_{Ba} = 2.0$7, and bolometric magnitudes of $M_{Ba} = 2.0$8 and $M_{Ba} = 2.0$9 mag, respectively (Erdem et al., 7 Aug 2025).

The orbital separation of the close pair is $M_{Bb} = 1.8$0, equivalent to $M_{Bb} = 1.8$1 AU. The mean orbital eccentricity is reported as $M_{Bb} = 1.8$2, obtained as an average across photometry, spectroscopy, and timing, and the inclination is $M_{Bb} = 1.8$3. The distance is given as $M_{Bb} = 1.8$4 pc in the $M_{Bb} = 1.8$5 band and is stated to be consistent with Gaia DR3 at $M_{Bb} = 1.8$6 pc. Interstellar reddening is $M_{Bb} = 1.8$7 mag from SED analysis, and the system age is approximately $M_{Bb} = 1.8$8 Myr, matched to Geneva and Padova isochrones at $M_{Bb} = 1.8$9 (Erdem et al., 7 Aug 2025).

These values place the A subsystem among the comparatively well-characterized young eclipsing binaries for which masses, radii, temperatures, geometry, and distance can be jointly inferred. The paper’s emphasis on agreement among light-curve fitting, radial-velocity modelling, SED fitting, Gaia astrometry, and evolutionary tracks suggests that the derived parameters are internally consistent rather than dependent on a single diagnostic.

4. Pulsational behaviour

The TESS light curves reveal low-amplitude oscillations described as irregular and unrepetitive. Frequency analysis shows multiple significant peaks extending from $a = 0.179 \pm 0.012$0 to $a = 0.179 \pm 0.012$1, together with lower frequencies around $a = 0.179 \pm 0.012$2–$a = 0.179 \pm 0.012$3 (Erdem et al., 7 Aug 2025).

The preferred interpretation in the published study is that the secondary star Ab is probably the source of $a = 0.179 \pm 0.012$4 Scuti-type oscillations. This is based on placement in the HR diagram and period-orbit analysis, which are stated to favor Ab as lying in the $a = 0.179 \pm 0.012$5 Sct instability strip. The primary Aa is instead located in the SPB region and might show $a = 0.179 \pm 0.012$6-mode pulsations, although the dominant interpretation attributes the observed variability mainly to the secondary. The B component cannot be cleanly separated in TESS because of spatial-resolution limitations, but the reported systematics still favor Ab as the $a = 0.179 \pm 0.012$7 Sct pulsator (Erdem et al., 7 Aug 2025).

This pulsational diagnosis is relevant beyond variability classification. In a young multiple system with significant third light and unresolved companions, mode attribution is often ambiguous. Here the combination of HR-diagram placement, orbital context, and frequency content narrows the likely origin of the oscillations to the secondary. A plausible implication is that NO Puppis may be useful in studies that connect eclipsing-binary constraints with $a = 0.179 \pm 0.012$8 Scuti phenomenology, although the paper also stresses the irregular and unrepetitive character of the signal.

5. Rotation, synchronism, and apsidal behaviour

Rotational analysis in the primary was based on the He I $a = 0.179 \pm 0.012$9 profile. The measured projected rotational velocity is $P = 101.3 \pm 3.8$0 km s$P = 101.3 \pm 3.8$1, while the synchronous value calculated from the orbital period, primary radius, and inclination is $P = 101.3 \pm 3.8$2 km s$P = 101.3 \pm 3.8$3. These values agree within uncertainties, and the published interpretation is that the primary is rotating synchronously, or very nearly synchronously, with the mean orbital period (Erdem et al., 7 Aug 2025).

For the secondary, the evidence is described more cautiously: pseudo-synchronism is plausible, and the overall discussion supports nearly synchronous rotation for Aa and pseudo- rather than true synchronization for Ab. This distinction matters because the orbit is eccentric. In eccentric systems, exact synchronization with the mean orbital period and pseudo-synchronism near periastron are dynamically different states, and the paper’s formulation explicitly preserves that difference (Erdem et al., 7 Aug 2025).

The timing analysis also identifies rapid apsidal motion. The summary table gives an apsidal motion period of $P = 101.3 \pm 3.8$4 years. Since the orbit solution was constrained using 53 times of minima together with photometric and spectroscopic data, apsidal precession is not a secondary correction but part of the fundamental orbital description. This makes NO Puppis a system in which orbital shape, precession, and rotation must be analysed together rather than separately (Erdem et al., 7 Aug 2025).

6. The eccentricity problem and dynamical interpretation

One of the central features of NO Puppis is the retention of significant eccentricity in a very close binary. The reported orbital eccentricity, $P = 101.3 \pm 3.8$5, is characterized as unexpected for a short-period system with $P = 101.3 \pm 3.8$6 d and age $P = 101.3 \pm 3.8$7 Myr, because standard tidal arguments would tend to predict efficient circularization. The paper specifically notes that, for this system’s $P = 101.3 \pm 3.8$8, Zahn (1977) tidal evolution theory predicts circularization within several Myr, yet the eccentricity remains substantial (Erdem et al., 7 Aug 2025).

Several explanations are considered. First, the system may be near a boundary case, since the calculated $P = 101.3 \pm 3.8$9 is described as only marginally inside the range for efficient circularization. Second, the presence of the wide Ba,Bb subsystem enables Kozai-Lidov cycles, which can periodically pump the eccentricity of the close pair and thereby counteract tidal damping. The paper states that analytic estimates and numerical simulations give Kozai-Lidov timescales of $e = 0.054 \pm 0.041$0–$e = 0.054 \pm 0.041$1 yr, shorter than the system age and therefore consistent with episodic eccentricity growth (Erdem et al., 7 Aug 2025).

A third possibility raised in the discussion is past dynamical evolution associated with membership in a young association, including possible loss of prior components or interactions with other cluster members. The paper notes that close binary-binary or triple encounters can contribute to maintaining eccentricity, and that escape from the association may help preserve the current parameters. These mechanisms are presented as possible contributors rather than uniquely established causes (Erdem et al., 7 Aug 2025).

The broader significance of the eccentricity problem is methodological as well as astrophysical. NO Puppis couples a well-measured short-period eclipsing orbit to a wider hierarchical environment, so its present state tests whether conventional tidal evolution alone is adequate. This suggests that the system is most profitably interpreted not as an isolated binary, but as a dynamically coupled multiple in which tides, apsidal motion, synchronization, and secular perturbations operate simultaneously.

7. Status as a benchmark young multiple system

The published study presents NO Puppis as a rare, well-characterized, young, close, and eccentric eclipsing binary within a hierarchical quadruple configuration. Its observational definition rests on the combined use of TESS, BVR photometry, high-resolution spectroscopy, and visual astrometry, and the resulting parameter set is sufficiently detailed to support discussion of stellar structure, pulsation, spin-orbit coupling, and dynamical evolution within one system (Erdem et al., 7 Aug 2025).

Several aspects make the system especially notable. The absolute parameters of Aa and Ab are reported with relatively small uncertainties; the secondary appears to be the source of $e = 0.054 \pm 0.041$2 Scuti-type oscillations; the primary’s He I $e = 0.054 \pm 0.041$3 profile supports synchronous rotation; and the orbit remains appreciably eccentric despite its small separation. The coexistence of these features in a young multiple system gives NO Puppis unusual leverage as an empirical test case for binary and multiple-star evolution (Erdem et al., 7 Aug 2025).

A plausible implication is that future work on NO Puppis will focus less on basic detection and more on reconciliation of its orbital eccentricity with tidal theory, on refinement of the architecture of the wider system, and on disentangling pulsational contributions from unresolved components. Within the current literature, however, its defining status is already clear: NO Puppis is a young eclipsing binary in a hierarchical multiple system whose accurately derived absolute parameters highlight a dynamically non-trivial evolutionary state.