2024MNRAS.527.8991S

In this study, we present an analysis of over 34 yr of observational data from CAL 87, an eclipsing supersoft X-ray source. The primary aim of our study, which combines previously analysed measurements as well as unexplored publicly available data sets, is to examine the orbital period evolution of CAL 87. After meticulously and consistently determining the eclipse timings, we constructed an O - C (observed minus calculated) diagram using a total of 38 data points. Our results provide confirmation of a positive derivative in the system's orbital period, with a determined value of $\dot{P}=+\, 8.18\pm 1.46\times 10^{-11}$ s s^–1. We observe a noticeable jitter in the eclipse timings and additionally identify a systematic delay in the X-ray eclipses compared to those observed in longer wavelengths. We discuss the interplay of the pertinent factors that could contribute to a positive period derivative and the inherent variability in the eclipses.