Six occultations of stars by the coma of comets 78P/Gehrels 2, 69P/Taylor, and 81P/Wild 2 have been observed using the 82cm IAC-80 telescope at Teide Observatory (Canary Island, Spain). Time series CCD images using R broadband filter were done. No star brightness extinction was detected even for the closest approach (400 Km from the nucleus). A detailed explanation of the reduction procedure and a discussion on some effects that can produce ``false" drops of the measured star magnitud is presented.