QZ Serpentis: A Dwarf Nova with a 2 Hour Orbital Period and an Anomalously Hot, Bright Secondary Star11Based in part on observations obtained at the MDM Observatory, operated by Dartmouth College, Columbia University, Ohio State University, and the University of Michigan.

Abstract

ABSTRACTWe present spectroscopy and time‐series photometry of the dwarf nova QZ Ser. The spectrum shows a rich absorption‐line spectrum of type K4 ± 2. K‐type secondary stars are generally seen in dwarf novae with orbital periods Porb ∼ 6 hr, but in QZ Ser the absorption radial velocities show an obvious modulation [semiamplitude 207(5) km s-1] at Porb = 119.752(2) minutes, much shorter than typical for such a relatively warm and prominent secondary spectrum. The Hα emission‐line velocity is modulated at the same period and roughly opposite phase. Time‐series photometry shows flickering superposed on a modulation with two humps per orbit, consistent with ellipsoidal variation of the secondary’s light. QZ Ser is a second example of a relatively short period dwarf nova with a surprisingly warm secondary. Model calculations suggest that the secondary is strongly enhanced in helium and had already undergone significant nuclear evolution when mass transfer began. Several sodium absorption features in the secondary spectrum are unusually strong, which may indicate that the present‐day surface was the site of CNO‐cycle hydrogen burning in the past.