| Abstract: | For the first time, BVIc observations of the Cepheid V811 Oph were obtained by using a new 60‑cm telescope at the Caucasus Mountain Observatory, Sternberg Astronomical Institute, Moscow State University (SAI MSU). An $$O{-}C$$ diagram covering a time span of 124 years was plotted using photometry from old photographic and modern sky surveys. The diagram has a parabola shape, which made it possible to determine for the first time the quadratic elements of the brightness variations. Furthermore, it made it possible to calculate the rate of an evolutionary decrease in the period of V811 Oph $$dP{\text{/}}dt = - 0.00375( \pm 0.00177)$$ s/year, which is consistent with the theoretical calculation results for the second intersection of the instability strip if this Cepheid is a classical one. The pulsation stability test proposed by Lombard and Koen confirmed the actual decrease in the period. |
