1998A&A...333..205F

UBV photometry and moderate resolution Hα spectrophotometry of the evolved binary system HD 185510 (sdB + K0 III), performed at Catania Astrophysical Observatory, is presented and discussed. The spectrophotometric data were collected in 1991, 1993, and 1994, while the photometric light curves were obtained in 1993, 1994 and 1995. From the B and V photometry we determine a new photometric rotational period of 26.23d, confirming the asynchronous rotation of the cool giant component. The spectroscopic data confirm the vsin i value of 15km/s measured by Fekel et al. (1993AJ....106.2370F) and clearly reveal a filled-in Hα line with appreciable variations. The excess emission of the line, observed at any orbital phase, is found to be anticorrelated with the V light curve and is primarily ascribed to the chromospheric activity on the cool star. The primary total eclipse is clearly visible in the U band, but undetectable in the V band. From the U observations we determined a total duration of the primary eclipse (from 1^st to 4^rd contact) of 1.3883d, with the ingress lasting only 27 minutes. This new accurate monitoring and timing of the eclipse allowed us to improve the system solution which leads to R_C=8.8R_☉, T_C=4800K, R_H=0.11R_☉, T_H=30000K for the cool and hot star respectively. The evolution of HD 185510B is discussed also in relation to the evolutionary status of HD 185510A and the synchronization time scale. HD 185510B is probably a sdB near the zero age extended horizontal branch, resulting from an enhanced mass loss in late case B or case A mass exchange with a possible common envelope phase. A small amount (15-20%) of mass loss from the system which can account for the strong IR excess is suggested.