HT Cas is a dwarf nova with a binary period of just 106 minutes. The eclipse of the white dwarf and accretion disk by the red giant can be spectacularly deep when outbursts occur. Each eclipse lasts just 12 minutes and high speed photometry can yield impressive light curves which help us to understand the astrophysics envolved in these highly energetic binary stars.

This animated gif  image shows HT Cas undergoing a dramatic fade and recovery of two magnitudes in a very short space of time. Each image ( a total of eight in the series)  is a 40 sec exposure with the 50cm telescope and CCD.

Magic Moments with HT Cas

I must admit that I have never been much of a variable star observer.  I had seen Algol go through its eclipses many times over a period of  several hours, but I had never been attracted to observing variables over time spans of weeks or months, so as to obtain those impressive light curves from many hundreds of observations.

There is however a particular class of variable stars that do exhibit exciting fluctuations in their light ouput measurable in matters of seconds rather than hours - that of the eclipsing dwarf novae.
 
A telephone call from Dr. Tim Naylor from Keele, alerted me to the fact that the dwarf nova HT CAS was undergoing a rare outburst and could I get a series of CCD images of it when it undergoes its eclipse. This is an intriguing binary star, as I understand it a red giant that rotates around a white dwarf companion in a period of a little under two hours.

It seems hard to visualize the gravitational forces at work here, but from our vantage point of this system from Earth, the red giant actually eclipses the white dwarf companion with each eclipse lasting just twelve minutes. Unfortunately these stars are normally very faint at mag 16.4, but every so often the white dwarf undergoes an outburst caused by an instability of matter falling onto its accretion disk from its companion.

The last such outburst occurring in 1985. At these outbursts the star brightens to mag 13, which is an easy target for CCD cameras. The project envolved obtaining images throughout the eclipse at a high time resolution of ideally 6 second intervals.
By so doing, CCD photometry would enable us to obtain a light curve which may well shed light on the finer structure of the star and accretion disk in outburst, any humps in the curve would point to hotspots in the system known superhumps.
 
Well that sounds fine in theory but at that rate of data collection you have to keep your wits about you! The main problem of course lies with computer memory and storage. Each of my images requires 260k, so over a period of twelve minutes I would capture 120 images requiring 31 Mb of memory. Time did not allow me to do the necessary disk housekeeping so I decided to monitor the eclipse at 40 sec intervals. By keeping the camera on continuous mode I found that I had time to type in a time
stamp and file name and save each image to disk before the next image arrived.
 
Because the period of the binary is two hours you get several chances each night to observe the eclipse, the times given for the evening of  19th/20th November 1995 were thus:

20:10
21:56
23:43
01:28
03:14
05:00
06:47
 
Unfortunately I didn't have an accurate finder chart so spent half an hour or so until I felt confident that I was on target. I did a practice run on the 2156h eclipse, all went well and to use "HST speak" - no anomalies were noted! I then proceeded to dark frame each image and while reloading each image it was very apparent how quickly the light of HT Cas was changing! At one minute intervals the changing magnitude was easily visible in the raw image. To observe a star change three magnitudes in a
matter of six minutes is really quite an exciting event to witness and adds a completely new dimension to the art of variable star observing.

Time  dMagStar A   V Mag

2342     2.81      14.71
2343     2.86      14.76
2344     2.49      14.39
2345     1.05      12.95
2346     0.89      12.79
2347     0.78      12.68
2348     0.67      12.57
2349     0.65      12.55
2350     0.69      12.59
2351     0.73      12.63
2352     0.71      12.61
2353     0.73      12.63
2354     0.72      12.62
 
These represent the star coming out of mid-eclipse at 2343h brightening from mag 14.71 to mag 12.55 in a matter of six minutes. This outburst was short lived however and the following night it had faded a further magnitude. The Hubble Space Telescope was due to image HT Cas on the 23rd Nov. I wonder what results that will give?
 
D. Strange
Worth Hill Observatory
20th Nov. 1995