  High spatial resolution is the key for the understanding various
astrophysical phenomena. But even with the future E-ELT, single dish
instruments are limited to a spatial resolution of about 4 mas in the visible.
For the closest objects within our Galaxy most of the stellar photosphere
remains smaller than 1 mas. With the success of long baseline interferometry
these limitations were soom overcome. Today low and high resolution
interferometric instruments on the VLTI and CHARA offer an immense range of
astrophysical studies. Combining more telescopes and moving to visible
wavelengths broadens the science cases even more. With the idea of developing
strong science cases for a future visible interferometer, we organized a
science group around the following topics: pre-main sequence and main sequence
stars, fundamental parameters, asteroseismology and classical pulsating stars,
evolved stars, massive stars, active galactic nuclei (AGNs) and imaging
techniques. A meeting was organized on the 15th and 16th of January, 2015 in
Nice with the support of the Action Specific in Haute Resolution Angulaire
(ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange
Laboratory and the Observatoire de la Cote d'Azur, in order to present these
cases and to discuss them further for future visible interferometers. This
White Paper presents the outcome of the exchanges.
  This book is dedicated to the memory of our colleague Olivier Chesneau who
passed away at the age of 41.
