Optical Photometry of the Type Ia Supernova 1999ee and the Type Ib/c Supernova 1999ex in IC 5179

We present UBVRIz light curves of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both located in the galaxy IC 5179. SN 1999ee has an extremely well-sampled light curve spanning from 10 days before B_max through 53 days after peak. Near maximum, we find systematic differences of ~0.05 mag in photometry measured with two different telescopes, even though the photometry is reduced to the same local standards around the supernova using the specific color terms for each instrumental system. We use models for our bandpasses and spectrophotometry of SN 1999ee to derive magnitude corrections (S-corrections) and remedy this problem. This exercise demonstrates the need of accurately characterizing the instrumental system before great photometric accuracies of Type Ia supernovae can be claimed. It also shows that this effect can have important astrophysical consequences, since a small systematic shift of 0.02 mag in the B-V color can introduce a 0.08 mag error in the extinction-corrected peak B magnitude of a supernova and thus lead to biased cosmological parameters. The data for the Type Ib/c SN 1999ex present us with the first ever observed shock breakout of a supernova of this class. These observations show that shock breakout occurred 18 days before B_max and support the idea that Type Ib/c supernovae are due to the core collapse of massive stars rather than thermonuclear disruption of white dwarfs.