Spectroscopic and photometric observations of a Z Cam-type dwarf nova, AT Cancri, in standstill

We performed quasi-simultaneous spectroscopic and photometric observations of a Z Cam star, AT Cnc, in standstill in 1997 February. V-band time-resolv ed photometry marginally revealed small-amplitude (similar to 0.01 mag) osc illations with period candidates of 0.132 d, 0.124 d, 0.142 d, and 0.249 d. Low-resolution spectra show Balmer lines in emission and absorption, and N a D absorption. From the radial-velocity variation of H alpha, we deduced t he system parameters: the orbital period = 0.2011(+/-0.0006) d, the semiamp litude = 80(+/-4) km s(-1), the systemic velocity = 11(+/-3) km s(-1), the primary mass = 0.9(+/-0.5) M.. The phase of the velocity variation of H alp ha is very close to that of Na D. There are two possibilities: the H alpha emission line radiated (a) from the secondary star, or (b) from the accreti on disk. The inclination is estimated to be 17 degrees (+/- 5 degrees) and 36 degrees (+/- 12 degrees) based on the respective possibilities. Doppler map drawn under possibility (a) revealed on a broadly extended component in the (-V-x, -V-y) region. In the Doppler map drawn under the possibility (b ), the peak exists on the irradiated side of the Doppler position of the se condary and the (+V-x, V-y) region. We discovered an absorption component i n the blue wing at H alpha. The P Cyg profile appeared during a limited per iod in its orbital phase, which is interpreted to be caused by transient ma ss ejection episodes.