The sensitivity of the COS FUV detectors declines with time and is monitored with regular observations of spectrophotometric standard stars (the white dwarfs WD 0308-565 and GD71). The slope of the Time Dependent Sensitivity (TDS) depends on grating, and detector segment, and currently varies between ~1% per year and ~5% per year, with steeper declines at longer wavelengths (see http://www.stsci.edu/hst/cos/performance/sensitivity/). The TDS slope for Segment A of the FUV channel is shallower than currently modeled in the TDS reference file. A new COS/FUV TDS reference file, 0561933ll_tds.fits, that takes into account the new TDS slopes was delivered on on 9 May 2016. This new reference file applies to all the COS FUV settings with the exception of G130M/1055 and 1096, for which the TDS reference file is left unchanged. The absolute and relative flux accuracy is within 5% and 2%, respectively, for all modes except for the G130M 1055 and 1096 CENWAVE settings, and for G140L data on Segment B. The absolute flux accuracy for the 1055/1096 settings is 20% while it is 40% for G140L/1280 FUVB (see the December 2013 STAN and the COS Data Handbook section 4.3.1).
Users concerned with the absolute or relative flux calibration of COS FUV spectra taken after October 2013 should re-retrieve their data from the Hubble Space Telescope Archive, hosted by MAST, at http://archive.stsci.edu/hst/.