2022A&A...663A..62P

Blue large-amplitude pulsators (BLAPs) form a small group of hot objects pulsating in a fundamental radial mode with periods of the order of 30 min. Proposed evolutionary scenarios explain them as evolved low-mass stars: ∼0.3 M_☉ shell-hydrogen-burning objects with a degenerated helium core, more massive (0.5-0.8) M_☉ core-helium-burning stars, or ∼0.7 M_☉ surviving companions of type Ia supernovae. Therefore, their origin remains to be established. Using data from Transiting Exoplanet Survey Satellite, we discovered that HD 133729 is a binary consisting of a late B-type main-sequence star and a BLAP. The BLAP pulsates with a period of 32.37 min decreasing at a rate of ( - 7.11 ± 0.33) x 10^–11. The light curve is typical for BLAPs, but it shows an unusual 40-s drop at the descending branch. Due to light dilution by a brighter companion, the observed amplitude of pulsation is much smaller than in other BLAPs. From available photometry, we derived times of maximum light, which revealed the binary nature of the star via an O-C diagram. The diagram shows variations with a period of 23.08433 d that we attribute to the light-travel-time effect in the system. The analysis of these variations allowed us to derive the spectroscopic parameters of the BLAP's orbit around the binary's centre of mass. The presence of a hot companion in the system was confirmed by the analysis of its spectral energy distribution, which was also used to place the components in the Hertzsprung-Russell diagram. The obtained position of the BLAP fully agrees with the location of the other members of the class. With the estimated V ≃ 11 mag and the Gaia distance of less than 0.5 kpc, the BLAP is the brightest and the nearest of all known BLAPs. It may become a key object in the verification of the evolutionary scenarios for this class of variables. We argue that low-mass progenitors of the BLAP are excluded if the components are coeval and no mass transfer between the components took place.