We study the optical counterparts of ultraluminous X-ray sources NGC 4559 X-10 and NGC 4395 ULX-1 .
Their absolute magnitudes , after taking the reddening into account , are M _ { V } \approx - 5.3 and M _ { V } \approx - 6.2 , respectively .
The spectral energy distribution of the NGC 4559 X-10 counterpart is well fitted by a spectrum of an F-type star , whereas NGC 4395 ULX-1 has a blue power-law spectrum .
Optical spectroscopy of NGC 4395 ULX-1 has shown a broad and variable HeII \lambda 4686 emission , which puts this object in line with all the other spectrally-studied ULXs .
Using the Swift archival X-ray data for NGC 4395 ULX-1 , we have found a period of 62.8 \pm 2.3 days .
The X-ray phase curve of the source is very similar to the precession curve of SS 433 .
The optical variation of the counterpart ( between two accurate measurements ) amounts to 0.10 mag .
Analyzing the absolute magnitudes of 16 well-studied ULX counterparts one may suggest that as the original accretion rate decreases ( but nevertheless remains supercritical ) , the optical luminosity of the wind becomes dimmer and the donor star dominates .
However , an observational bias may also influence the distribution .