Context : Recent investigations revealed very peculiar properties of blue horizontal branch ( HB ) stars in \omega Centauri , which show anomalously low surface gravity and mass compared to other clusters and to theoretical models . \omega Centauri , however , is a very unusual object , hosting a complex mix of multiple stellar populations with different metallicity and chemical abundances .

Aims : We measured the fundamental parameters ( temperature , gravity , and surface helium abundance ) of a sample of 71 blue HB stars in M22 , with the aim of clarifying if the peculiar results found in \omega Cen are unique to this cluster .
M22 also hosts multiple sub-populations of stars with a spread in metallicity , analogous to \omega Cen .

Methods : The stellar paremeters were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra .
From these parameters , the mass and reddening were estimated .

Results : Our results on the gravities and masses agree well with theoretical expectations , matching the previous measurements in three “ normal ” clusters .
The anomalies found in \omega Cen are not observed among our stars .
A mild mass underestimate is found for stars hotter than 14 000 K , but an exact analogy with \omega Cen can not be drawn .
We measured the reddening in the direction of M22 with two independent methods , finding E ( B - V ) = 0.35 \pm 0.02 mag , with semi-amplitude of the maximum variation \Delta ( E ( B - V ) ) = 0.06 mag , and an rms intrinsic dispersion of \sigma ( E ( B - V ) ) = 0.03 mag .

Conclusions :