### Roy Andrew Axelsen and Tim Napier-Munn

#### Astronomical Association of Queensland; American Association of Variable Star Observers; Variable Stars South

Our understanding of the delta Scuti star AD CMi has evolved as observational data since 1959 has gradually accumulated to present a more or less complete picture of its behaviour. Its period is approximately 2 hr 57 min. Data obtained between 1959 and 1992 revealed an O-C (observed minus computed) diagram described by a quadratic function which suggested that the period of the star was increasing slowly at a constant rate. However, the addition of further data, to 2006, revealed a more complicated O-C diagram with a quasi-sinusoidal shape, described by a combined quadratic and trigonometric function. The data implied that, although the pulsational period of the star was indeed increasing at a slow constant rate, the O-C diagram was modulated by the light time effect of a binary system. Data published in 2007 included 81 times of maximum (TOM). We performed photoelectric photometry on one night in 2011 and DSLR photometry during 7 nights in January and February 2016, by which time 9 years had elapsed since the last publication of data in a refereed journal. The literature to 2007 contained 81 TOM, subsequent literature and the AAVSO international database added another 28, and 9 TOM were contributed by our own observations, yielding a total of 118 TOM, by far the largest database in the literature on this star. Assuming a linear ephemeris, the period of AD CMi was calculated to be 0.122974511 (4) d, almost identical to that quoted in earlier literature. Analysis of the O-C diagram confirmed the results of previous authors, and updated most of the coefficients of the function fitted to the diagram. The values of all of the coefficients were statistically significant. We calculated the pulsational period of AD CMi to be increasing at a constant rate of dP/dt = 6.17 (+/- 0.75) x 10-9 d yr-1 or dP/Pdt = 5.01 (+/- 0.61) x 10-8 yr-1. Because estimates of the period of the binary system range from 27.2 y to 42.8 y across four different published papers, it may be decades before an accurate determination of the orbital period can be made from photometric data.