I-band period–luminosity relation ( P in days) for classical fundamental mode Cepheids as defined by the catalog of OGLE III Cepheids in the LMC (Soszynski et al. 2008). The position of our binary system is marked by the upper filled circle (out-of-eclipse total brightness of the system, including the faint third star in the light), and the position of CEP-1812 (its mean brightness, freed from the contributions coming from the binary companion and the faint third star) is indicated by the lower circle. The Cepheid falls almost exactly on the ridge line of the period–magnitude relation indicating that CEP-1812 is a classical Cepheid.