The Type Ic SN 1990B in NGC 4568

We present a study of the Type Ic supernova (SN) 1990B that includes most o f the observations obtained from around the world. The combined data set co mprises 84 BV(RI)(c) photometric points spanning approximately 360 days aft er maximum light and 14 spectra from 5 up to similar to 150 days after maxi mum light. In contrast to other Type Ic SNe, SN 1990B did not display a wea k but distinct He I lambda 5876 line indicating that its He content was sma ller or that the He layers were rather effectively shielded from the radioa ctive matter in the ejecta. The behavior of the Na I D line, however, sugge sts that He I lambda 5876 was blended with it. SN 1990B appeared on a sharp ly varying background that complicates the usual techniques of digital phot ometry. In order to do unbiased photometry, we modeled and subtracted the b ackground of each image with the SN using images of NGC 4568 taken similar to 2500 days after the explosion, when SN 1990B had faded beyond detection. We compare the performance of standard point-spread function fitting photo metry of the SN in the images with and without the background of the parent galaxy and find the results to differ systematically at late times. The ph otometry done on the images with the background light of NGC 4568 subtracte d shows the light curves of SN 1990B to be of the slow Type Ic variety, wit h a slope steeper than that of the Type Ib SN 1983N or the Type II transiti on (Type IIb) SN 1993J but slower than that of the Type Ic SN 1994I. We est imate the reddening by foreground matter in the Galaxy and NGC 4568 and com pute BV(RI)(c) light curves spanning similar to 110 days after maximum ligh t.