An ultraviolet study of the short-period binary OO Aquilae

OO Aq1 is a rare W UMa-type eclipsing binary in which the two solar-type st ars may have only recently evolved into contact. The binary has an unusuall y high mass ratio (0.84) and a relatively long orbital period (0.506 days) for its spectral type (mid-G). Twelve ultraviolet spectra of OO Aq1 were ob tained in 1988 with the International Ultraviolet Explorer satellite, inclu ding a series of consecutive observations that cover nearly a complete orbi tal cycle. Chromospheric activity is studied by means of the Mg II h and k emission at 2800 Angstrom. The Mg II emission is found to vary, even when t he emission is normalized to the adjacent continuum flux. This variation ma y be correlated with orbital phase in the 1988 observations. It also appear s that the normalized Mg II emission varies with time, as seen in spectra o btained at two different epochs in 1988 and when compared with two spectra obtained several years earlier. The level of chromospheric activity in OO A q1 is less than that of other W UMa-type binaries of similar colors, but th is is attributed to its early stage of contact binary evolution. Ultraviole t light curves were composed from measurements of the ultraviolet continuum in the spectra. These were analyzed, along with visible light curves of OO Aq1, to determine the system parameters. The large wavelength range in the light curves enabled a well-constrained Dt to a cool spot in the system.