BRITE nascent binaries

Abstract: Nascent binaries (NBs) are binary systems with very low mass ratios, less than ~0.2, in which the more massive component is an O- or B-type main-sequence star, while the secondary is a star contracting onto the main sequence. NBs are of interest because they can help to understand the formation of small-mass ratio systems and shed light on the origin of low-mass X-ray binaries, millisecond pulsars and type Ia supernovae. In photometry, short-period NBs show a strong irradiation effect due to the large difference between the effective temperatures of the components and the strong irradiation of a cool secondary by a hot primary. In spectroscopy, they usually appear as single-lined spectroscopic binaries. In the present paper, we summarize the status of our knowledge of Galactic nascent binaries and characterize two new members of this group, c2 Sco and V390 Pup, for which photometric data were obtained by the BRIght Target Explorer (BRITE) nano-satellite mission.