Optical Photometry of the Type I[CLC]a[/CLC] Supernova 1999[CLC]ee[/CLC] and the Type I[CLC]b[/CLC]/[CLC]c[/CLC] Supernova 1999[CLC]ex[/CLC] in IC 5179

We present UBVRIz lightcurves of the Type Ia SN 1999ee and the Type Ib/c SN1999ex, both located in the galaxy IC 5179. SN 1999ee has an extremely wellsampled lightcurve spanning from 10 days before Bmax through 53 days afterpeak. Near maximum we find systematic differences ~0.05 mag in photometrymeasured with two different telescopes, even though the photometry is reducedto the same local standards around the supernova using the specific color termsfor each instrumental system. We use models for our bandpasses andspectrophotometry of SN 1999ee to derive magnitude corrections (S-corrections)and remedy this problem. This exercise demonstrates the need of accuratelycharacterizing the instrumental system before great photometric accuracies ofType Ia supernovae can be claimed. It also shows that this effect can haveimportant astrophysical consequences since a small systematic shift of 0.02 magin the B-V color can introduce a 0.08 mag error in the extinction correctedpeak B magnitudes of a supernova and thus lead to biased cosmologicalparameters. The data for the Type Ib/c SN 1999ex present us with the first everobserved shock breakout of a supernova of this class. These observations showthat shock breakout occurred 18 days before Bmax and support the idea that TypeIb/c supernovae are due to core collapse of massive stars rather thanthermonuclear disruption of white dwarfs.Comment: 55 pages, 15 figures, accepted by the Astronomical Journa