RX J15542+2721: An Apparently Magnetic Cataclysmic Variable with a 2.53 Hour Orbital Period

Abstract

ABSTRACTTime‐resolved spectroscopy and photometry of the cataclysmic variable (CV) star RX J15542+2721 show it to have an orbital period of 2.53 hr, putting it in the middle of the so‐called CV period gap from 2 to 3 hr. Observations taken 2001 March show it in a low state with relatively weak emission lines and a strong contribution from a secondary star near type M4. Contemporaneous I‐band differential photometry shows ellipsoidal variations. In 2001 May the emission lines were stronger and of higher excitation, with prominent He ii λ4686, the secondary star less visible, and the ellipsoidal variations masked by flickering. By 2001 June the object had returned to a state similar to the March observations. RX J15542+2721 appears to be an AM Her–type magnetic CV. Archival plate material, although sparse, indicates that the object may spend most of its time close to minimum light. This, and a weak indication of a 2–3 hr “trough” in the histogram of AM Her star orbital periods, suggest that the mass transfer rate may be somewhat depressed among AM Her stars in this period range. The spectral type of the secondary and the orbital period are consistent with a distance of roughly 210 pc. A proper motion μ = 28 ± 4 mas yr−1 is found, corresponding to a transverse velocity ∼28 km s−1 at the nominal distance. The radial velocity variations of the M dwarf secondary are detected, and their phase is essentially as expected based on the ellipsoidal variations. The data suggest fairly typical masses for the system components, and a more detailed study might constrain the white dwarf mass rather accurately.