The following calibration format can be used directly in 
IRAF.digiphot.photcal.invertfit. The instrumental magnitudes
mU, mB, and mV are -2.5*log(counts/s)+25.

For the these images, the average parameters for UBV are the following:
Airmass---1.51, 1.49, 1.48
Integration time (s)---600, 500, 200
Pixel scale (arcsec)---0.27, 0.27, 0.27

begin UFIT
      status 0
      parameters 2
              u1
              u2
      derivatives   2
              0.1
              0.1
      values 2
             2.157223
             -0.1136566
      errors 2
            0.0054008
            0.01313

begin BFIT
      status 0
      parameters 2
              b1
              b2
      derivatives   2
              0.1
              0.1
      values 2
             0.079320
             -0.12177
      errors 2
            0.014319
            0.01818

begin VFIT
      status 0
      parameters 2
              v1
              v2
      derivatives   2
              0.1
              0.1
      values 2
             -0.05303964
             0.02208642
      errors 2
            0.0114193
            0.014448