HT Cas is a dwarf nova located in Cassiopeia (1h 10m 13.12s, +60d 04' 35.7"). Outbursts of HT Cas typically occur every year or two. During an outburst, it brightens from magnitude 16 to 12. The rise to full outburst takes only a day. During an outburst, the brightness will fluctuate by a few tenths of a magnitude in a regular pattern that is related to the orbital period of the system. The two stars in the HT Cas system orbit each other every 106 minutes. What makes HT Cas even more interesting is the fact that the orbital plane of the binary system is pointed towards Earth, which means we can see the dimmer main sequence star eclipse the brighter white dwarf and accretion disk every 106 minutes.
In November, 2010, HT Cas had a bright outburst. I observed this outburst for several nights with my 8” f/5 Newtonian in my backyard and the CVAS 16” f/7 Newtonian at Indian Hill Observatory. Below is a light curve of HT Cas made from 205 CCD images on the night of November 11, 2010. These images were 30 second exposures made with a ST-7XME + V filter and the CVAS 16” f/7 Newtonian. I performed differential photometry on this set of images using two nearby comparison stars. The comp stars’ differential brightness is plotted also to show that conditions were good during the time series. There is a gap in the observations just before the second eclipse. I had to stop imaging for 15 minutes to remove dew that was forming on the secondary mirror (I’ve got to get a dew heater for the 16” next year!). Despite the brief interruption, I obtained a good set of data clearly showing two eclipses 106 minutes apart as well as a pronounced orbital hump in between them. Click on the image for a larger version.
Below is short animation showing the first of two eclipses imaged on November 11, 2010. HT Cas is centered in the frame and a magnitude 13.9 star lies just below it.