On Fri, May 15, 2015 at 08:16:55PM -0400, Freelance Traveller wrote:
[ RV Tauri variables ]
> What would it be like to live on a world where the sun ranges from
> yellow to red and back over a period measured in months?

The biggest concern would be the star's heat output.  The averaged
luminosity of such stars can vary by a factor of about five from year
to year.  A planet having a median habitable temperature about such a
star would likely suffer freezing over the whole surface one year
(e.g. equatorial mean temperature -20 C), and hotter than boiling
water over most of the planet the next (equatorial mean 120 C).

Although in the long run this would make retaining surface water
impossible, such stars are only in this phase for a short time,
astronomically speaking -- merely thousands of years.  Oceans could
exist, which would act to moderate temperature extremes somewhat.  The
weather conditions driven by temperature variations would probably be
very much more powerful than Earth's, with incredible rainfall
possible if seas are present.

The natural atmosphere would be very unlikely to have useful amounts
of free oxygen.  Even in a universe like Traveller's where almost all
planets that could have life, do have life, a planet about such a star
has probably only been warm enough to support liquid water for less
than a million years.  It took life on Earth around a thousand million
years to create enough excess oxygen to get into the atmosphere.
Human usable air (even with respirators) would require some vigorous
handwaving or deliberate engineering.

All these effects could be moderated somewhat if the variable is a
more distant companion of a binary star system with the planet
orbiting the more sedate member of the pair, or if the star is only
just beginning to enter its variable phase.

- Tim