Five maxima of T Cephei

Here is a light curve, based on observations by SPA VSS members, that shows the five most recent maxima of the Mira type variable T Cephei:

T Cep

As is normal for Mira type variables, the brightness at maximum varies from cycle to cycle, with the 2017 peak being the faintest for quite a few years.

A particularly distinctive feature that has become more prominent in recent years has been the ‘pause’ near mag 8.0 during the rise to maximum.  From being quite brief during the rise to the 2013 and 2014 maxima, it extended to last for around 2 months in early 2015 and for nearly 3 months in early 2016.  There is more scatter around the ‘pause’ in the early 2017 light curve, but it does seem to have lasted for about 7 weeks – from late December to mid-February.

Given the extended length of these ‘pauses’ in recent years, it might be expected that these would cause the subsequent maxima to occur later than predicted.  However, the opposite seems to have been the case, with the brightening more than making up time after the ‘pause’.  The average period of T Cephei is usually listed as 389 days and so, on average, maxima would be expected to occur about 24 days later each year.  However, closer inspection of the light curve indicates that maxima occurred in late March 2013, early April 2014, late April 2015 and early May 2016.  The 2017 peak is somewhat flat-topped, making it harder to estimate the date of maximum, but the second week of May would seem most likely.  This would indicate that the first and fifth maxima were separated by no more than 50 days, indicating an average inter-peak gap of around 12 days – half that expected.

When will the next maximum occur?   It could be in late May 2018 (12 day gap), early June 2018 (24 day gap) or possibly several weeks later (longer gap to bring it back in line with the long-term average).  We will have to wait and see.