Irregular variables form a heterogeneous class of objects displaying slow to fast photometric variations with no obvious periodicities. The objects classified as L-type (also called slow irregular variables) are usually late-type giants (LB) or supergiants (LC) for which the variability is associated with pulsation in their atmospheres, while the variables designed as I are eruptives and the variability is caused by violent processes like flares that take place in their chromospheres or coronae. In addition, the I variables may be associated to nebulosity (IN) or may present rapid variability (IS).
Often, when a star is known to be variable, but no periodicity is found and no other information is available, it is classified as "irregular''. This is common in the case of the stars found to be variable in photographic patrols. More detailed studies may show a large fraction of misclassification among these objects. In fact, several classes of variables can display irregular photometric variability without being a "classical'' irregular variable of type L or I as defined in the General Catalogue of Variable Stars (Kukarkin et al. 1969). Cataclysmic variables and QSOs are good examples. In the past, a number of active galactic nuclei were initially classified as irregular variables, like the radio galaxy 3C120 (Penston 1968); the prototype of the "blazars'', BL Lac (Schmitt 1968); QSOs like V395 Her (Bond 1972). This has also happened with peculiar binary stars, like the X-ray binary HZ Her (Davidsen et al. 1972, and references therein) and the intermediate polar V1223 Sgr (Steiner et al. 1981). Systematic searches for nebulosities led to the discovery of extragalactic objects among previously classified irregular variables (Bond 1973). Systematic observations using UBV photoelectric photometry should be efficient in selecting peculiar galactic objects as well. This motivated us to do a photometric and spectroscopic study on irregular variables.
In this paper we present the results from photoelectric photometry of 616 stars. Follow-up spectroscopic observations (Part II) and data analysis (Part III) will be published elsewhere. A preliminary report on this project was published by Steiner et al. (1988).