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We identified two optical counterparts of brightest ultraluminous X-raySources (ULXs) in galaxies NGC5474 and NGC3627 (M66). The counterparts inHubble Space Telescope images are very faint, their V magnitudes are 24.7 ($M_V\approx -4.5$) and 25.9 ($M_V \approx -4.2$), respectively. NGC5474 X-1 changesthe X-ray flux more than two orders of magnitude, in its bright state it has$L_X \approx 1.6 \times 10^{40}$ erg/s, the spectrum is best fitted by anabsorbed power-law model with a photon index $\Gamma \approx 0.94$. M66 X-1varies in X-rays with a factor of ~2.5, its maximal luminosity being $2.0\times 10^{40}$ erg/s with $\Gamma \approx 1.7$. Optical spectroscopy of theNGC5474 X-1 has shown a blue spectrum, which however was contaminated by anearby star of 23 mag, but the counterpart has a redder spectrum. Among otherobjects captured by the slit are a background emission-line galaxy (z=0.359)and a new young cluster of NGC5474. We find that these two ULXs have largestX-ray-to-optical ratios of $L_X/L_{opt}$ ~ 7000 for NGC5474 X-1 (in its brightstate) and 8000 for M66 X-1 both with the faintest optical counterparts evermeasured. Probably their optical emission originates from the donor star. Ifthey have super-Eddington accretion discs with stellar-mass black holes, theymay also have the lowest mass accretion rates among ULXs such as in M81 X-6 andNGC1313 X-1.

Optical counterparts of two ULXs in NGC5474 and NGC3627 (M66)