Narrow-band CCD photometry of giant HII regions

We have obtained accurate CCD narrow-band Hbeta and Halpha photometry of giant H II regions (GEHRs) in M33, NGC 6822 and M101. Comparison with previous determinations of emission-line fluxes shows large discrepancies; their probable origins are discussed. Combining our new photometric data with global velocity dispersion (sigma) derived from emission linewidths, we review the L(HG)-sigma relation. A re-analysis of the properties of the GEHRs included in our sample shows that age spread and the superposition of components in multiple regions introduce a considerable spread in the regression. Combining the information available in the literature regarding ages of the associated clusters, evolutionary footprints on the interstellar medium, and kinematical properties of the knots that build up the multiple GEHRs, we find that a subsample-which we refer to as young and single GEHRs-do follow a tight relation in the L-sigma plane.