  We report results of an electron spin resonance (ESR) study of a spin-gap
antiferromagnet (C$_4$H$_{12}$N$_2$)(Cu$_2$Cl$_6$) (nicknamed PHCC) with
chlorine ions partially substituted by bromine. We found that up to 10% of
nominal doping the contribution of the random defects to the absorption spectra
remains at about 0.1% per copper ion, almost the same as in the pure system.
Instead, a particular kind of ESR absorption corresponding to gapless S=1
triplets is observed at low temperatures in samples with high nominal bromine
content x>5%. Increase of bromine concentration also leads to the systematic
broadening of ESR absorption line indicating reduction of the quasi-particles
lifetime.
