ORBITAL PERIODS OF THE DWARF NOVAE AR ANDROMEDAE, AM CASSIOPEIAE, ANDPY PERSEI

We present time-resolved spectroscopy of the dwarf novae AR And, AM Ca s, and PY Per. From velocities of H alpha emission lines, we determine orbital periods of 0.16302+/-0.00032 d (= 3.91 hr) for AR And and 0.1 5480+/-0.00017 d (= 3.72 hr) for PY Per. Our period determination for AR And resolves a daily cycle-count ambiguity in a previous determinat ion by Shafter et al. (1995, ApJ, 440, 853). No orbital period for PY Per has previously been published. The orbital period of AM Gas, deter mined here for the first time, is constrained over a 46-night baseline to one of several precise values near 0.165 d (= 3.96 hr), the best o f which are 0.16490+/-0.00001 d and 0.16551+/-0.00001 d. For PY Per, w e use a single direct CCD exposure to extend the magnitude sequence of Misselt (1996, PASP, 108, 146). A comparison with the Downes and Shar a atlas (1993, PASP, 105, 127) shows that PY Per had V similar to 19.8 when the Downes and Shara image was obtained, which is much fainter t han the minimum listed in the General Catalogue of Variable Stars [Kho lopov 1987 (Nakua, Moscow)]. Our averaged spectra of all appear typica l for dwarf novae, except that the Fe II lambda 5169 in AR And is unus ually strong and weak features at lambda 5230, lambda 5270, and lambda 5322 (which we attribute to Fe II) also appear.