RApid Temporal Survey (RATS) - II. Followup observations of four newly discovered short-period variables

The Rapid Temporal Survey (RATS) is a survey to detect objects whose optical intensity varies on time-scales of less than similar to 70 min. In our pilot data set taken with the Isaac Newton Telescope and the Wide Field Camera in 2003 November, we discovered nearly 50 new variable objects. Many of these varied on time-scales much longer than 1 h. However, only four objects showed a modulation on a time-scale of 1 h or less. This paper presents followup optical photometry and spectroscopy of these four objects. We find that RAT J0455 + 1305 is a pulsating (on a period of 374 s) subdwarf B star of the EC 14026 type. We have modelled its spectrum and determine T-eff = 29 200 +/- 1900 K and log g = 5.2 +/- 0.3 which locates it on the cool edge of the EC 14026 instability strip. It has a modulation amplitude which is one of the highest of any known EC 14026 star. Based on their spectra, photometric variability and their infrared colours, we find that RAT J0449 + 1756, J0455 + 1254 and J0807 + 1510 are likely to be SX Phe stars - dwarf delta Sct stars. Our results show that our observing strategy is a good method for finding rare pulsating stars.