While looking for some Mira type stars to observe I came across DM CMa which is classified as a Mira type variable star in the GCVS with a range of 13.5 to <16.0p. I decided to add this to my list of program stars and I started to observe this field in 2009. Within a short time, I caught a brightening of DM CMa on the 25th of March then it faded. Another brightening had been observed in April. That was a cycle of about 25 days, a bit strange for a Mira type variable which can have periods ranging from 80 to 1000 days.
Thomas Berthold from Sonneberg Observatory looked into the original discovery paper ANErg 12, N1, 1949. He wrote that the paper contains just a simple discovery note that
states the type as a Mira star and the brightness range of 14.5 to >16.5. He noted that Hoffmeister has evaluated the Sonneberg plates by a blink comparator and that he must have had the opinion that it is a Mira star based on his extensive experience and the apparent color of the star that was based on the evaluation of blue and yellow plates.
The spectral type of “M0:” originates from 1956ApJ…124..346C – Spectral Classification of Red Variables Along the Galactic Equator – Cameron, Donald; Nassau, J. J. who actually took the var type from the GCVS! DM CMa, therefore, seems to have had back then already the Mira var type classification but no spectral type in the GCVS. So the classification as Mira star very probably originates from the original study ref by Hoffmeister.
Taichi Kato noticed that DM CMa (GCVS type M) may be identified with a ROSAT source.
065245.6 -142145 (2000.0) 1RXS-F_J065245.6-142145 0.029 0.33 0.40. “There is no prominent red star as suggested from the literature spectral type of M0:. The more likely interpretation would be a cataclysmic (or related) object”.
DM CMa has been on my observing list for the past few seasons and my observations show the light curve below. There appear to be short outbursts around a few days and longer outbursts lasting over 10 days. The outburst range is from 14.0 to <15.8. It looks like a UG star to me. More observations will be obtained to get a better picture of the outburst behavior and to look for UGSU features.
Some photometry by Bernard Heathcote on DM CMa when bright at V= 14.78 (+/- 0.04) showed a (B-V)= 0.15 was obtained on April 20, 2009, which also doesn’t suggest a Mira type star.