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We study the optical counterparts of ultraluminous X-ray sources NGC4559 X-10and NGC4395 ULX-1. Their absolute magnitudes, after taking the reddening intoaccount, are $M_V \approx -5.3$ and $M_V \approx -6.2$, respectively. Thespectral energy distribution of the NGC4559 X-10 counterpart is well fitted bya spectrum of an F-type star, whereas NGC4395 ULX-1 has a blue power-lawspectrum. Optical spectroscopy of NGC4395 ULX-1 has shown a broad and variableHeII~$\lambda$4686 emission, which puts this object in line with all the otherspectrally-studied ULXs. Using the Swift archival X-ray data for NGC4395 ULX-1,we have found a period of $62.8\pm 2.3$ days. The X-ray phase curve of thesource is very similar to the precession curve of SS433. The optical variationof the counterpart (between two accurate measurements) amounts to 0.10 mag.Analyzing the absolute magnitudes of 16 well-studied ULX counterparts one maysuggest that as the original accretion rate decreases (but nevertheless remainssupercritical), the optical luminosity of the wind becomes dimmer and the donorstar dominates. However, an observational bias may also influence thedistribution.

Optical counterparts of two ultraluminous X-ray sources NGC4559 X-10 and NGC4395 ULX-1