1 Introduction

This work has been done within frames of the program of a search for new unique stellar objects in nearby galaxies, which we initiated in (Fabrika & Sholukhova 1995). By unique or anomalously active objects we mean SS433 -- a massive binary system, where a supercritical gas accretion onto relativistic star occurs, LBV stars (Luminous Blue Variables -- S Dor and P Cyg stars, Hubble-Sandage variables) that show superpowerful wind outflow, ${\rm B{[e]}}$ supergiants, bright WN stars. All these objects are young massive single or binary stars, brightest in the Galaxy. In their main spectral characteristics in optical range (blue continuum, bright ${\rm H}\alpha$ emission) these stars are similar to the object SS433, free from interstellar absorption. Another common property of these stars is they all are massive stars in critical stages of evolution. Progress in understanding of physics and evolution of these stars could be made if we find as many as possible of stars of such types or similar stars. Most such stars in the Galaxy are obscured from the observer because of considerable absorption of light. That is why it is important to search for such objects in nearby galaxies.

The most suitable for our purpose are massive spiral, nearly face-on galaxies, the best example is M33. It is important that this galaxy contains many young stars and it is well studied. Ivanov et al. (1993) (here and after IFM) have compiled a catalogue of OB stars up to $V=19\hbox{$.\!\!^{\rm m}$ }5$ in M33. Assuming the number of early massive stars in M33 to be about 2000, Fabrika & Sholukhova (1995) have estimated the expected number of SS433 type objects between 1 and 20. In that paper we have discussed basic criteria of a search for such objects in M33. These criteria are mainly associated with similarity to SS433. The object has to be 1) OB star and 2) ${\rm H}\alpha$ star, as SS433 itself is a very powerful ${\rm H}\alpha$ source. Additional criteria are 3) HeII $\lambda$4686 emission, 4) a radio nebula associated with candidate star, 5) a possible X-ray source, 6) brightness variability in V-band up to ${\rm 1^m}$ on a time scale from days to years. Besides, ${\rm H}\alpha$ line may be strongly variable. SS433 ${\rm H}\alpha$ line equivalent width changes from 100 to 1000 Å, while its FWHM is about 30 Å. Criteria 1, 2 and partly 6 are common to SS433 and LBV type stars, that is why it is worthwhile to make a concurrent search for such objects. The principal point is the existence of an early-type star with a strong (and possibly broad) emission line ${\rm H}\alpha$. That is why it seemed natural to extend the work and carry out a systematic search for all stars of the above-mentioned types on the basis of selection of objects by the criterion OB star+ ${\rm H}\alpha$ emission.

Based on all above enumerated criteria, Fabrika & Sholukhova (1995) have made a cross-correlation analysis and selected 171 stars to be candidates for unique objects (the list I). When making up this list we made a cross-correlation of coordinates of OB stars from IFM and star-like H$\alpha$ sources from Courtes et al. (1987). In the list I were also added 19 early stars with ${ V \le 18^m}$, which are visible on the H$\alpha$ images (Courtes et al. 1987) as stars brighter than the majority of OB stars of the same magnitude in V. The selection criterion was (Fabrika & Sholukhova 1995). This selection was based on visual estimates, therefore the list of OB- ${\rm H}\alpha$ stars in the list I (Table 6 in the quoted paper) is rather incomplete. In this paper we examine OB stars in H$\alpha$images on the base of photometry of all objects from IFM. This catalogue comprises 2112 stars up to $V=19\hbox{$.\!\!^{\rm m}$ }5$ which have been selected by the criteria U-V<0, U-B<0. When we started this work we used the 1st version of the catalogue (kindly made available by G. Ivanov prior to publication). We have supplemented it with new additional objects from published version. So we have studied H$\alpha$ images a total of 2332 blue stellar objects. It is not improbable that these objects include star-like nebula too, as well as compact groups of stars.