We verified for photometric stability a set of DA white dwarfs with Hubble Space Telescope magnitudes from the near-ultraviolet to the near-infrared and ground-based spectroscopy by using time-spaced observations from the Las Cumbres Observatory network of telescopes. The initial list of 38 stars was whittled to 32 final ones, which comprise a high-quality set of spectrophotometric standards. These stars are homogeneously distributed around the sky and are all fainter than r~16.5mag. Their distribution is such that at least two of them would be available to be observed from any observatory on the ground at any time at airmass less than 2. Light curves and different variability indices from the Las Cumbres Observatory data were used to determine the stability of the candidate standards. When available, Pan-STARRS1, Zwicky Transient Facility, and TESS data were also used to confirm the star classification. Our analysis showed that four DA white dwarfs may exhibit evidence of photometric variability, while a fifth is cooler than our established lower temperature limit, and a sixth star might be a binary. In some instances, due to the presence of faint nearby red sources, care should be used when observing a few of the spectrophotometric standards with ground- based telescopes. Light curves and finding charts for all the stars are provided.

Cone search capability for table J/ApJ/940/19/dawds (Gaia DR3 astrometry and photometry for the candidate spectrophotometric standard DA white dwarfs (Table 1) and spectroscopic and photometric parameters (Table 4))

Cone search capability for table J/ApJ/940/19/table2 (Log of the observations collected with the Goodman spectrograph on the 4m-SOAR telescope NOAO programs 2017A-0052 (PI: Olszewski))

Cone search capability for table J/ApJ/940/19/table3 (Log of the observations collected with LCO during programs LCO2016B-007, LCO2017AB-002, LCO2018A-002, LCO2018B-001, and LCO2019-B004 (PI: T. Matheson).)