HP Lyr - Possibly the Hottest RV Tau Type Object

We report Johnson's UBVRI photometric and optical spectroscopic observations of a long period variable HP Lyr which up to now has been considered to be an eclipsing binary with a period of 140 days. Its spectral type changes continuously from A2-3 at maxima to A7-F2 at minima. We propose that the brightness changes are caused by pulsation of the star with two periods: P1=69.35 days, and P2=2×P1=138.7 days. These periods decreased by more than 1% between 1960 and 1980. The spectral luminosity class corresponds to an A type supergiant Iab. HP Lyr is also the optical counterpart of the infrared source IRAS 19199+3950. Relatively high galactic latitude (b=+11°.7) and high radial velocity (-113 km/s) indicate that HP Lyr is an evolved, most likely post-AGB star. All these features suggest that this star is an RV Tau type object.