"Henri Wilson" HW@.... wrote in message
news
On 29 Mar 2007 10:25:26 -0700, "George Dishman"
wrote:

On 28 Mar, 01:50, HW@....(Henri Wilson) wrote:
...
Whether or not cepheids are really huff-puff stars doesn't matter. We
say their
brightness variations are due to c+v effects caused by their surfaces
moving in
and out. A brightess curve produced that way is likely to be similar to
that
for a star in elliptical orbit.

What ?????

For years you have been saying that Cepheids were plain
constant-luminosity stars and the variation was due to
c+v effects because they are in binary systems that have
not been recognised as such.

No I changed that opinion some time ago George. I accepted that the
presence of
harmonics in the brightness curves was pretty hard to explain on purely
'orbit'
grounds.
So it is quite likely that two factors are contributing to the brightness
curves of these stars. Their orbit motion and the huff-puffing of their
surfaces.

Given that you now accept the huff-puff nature, you
need to reconsider your justification for saying
that Cepheids that are currently thought of as
isolated might actually be part of a binary.

If you are now switching to say they are single stars, why
on Earth would your software be modelling binary systems
and restricting the solutions to Keplerian orbits when the
motion of the surface is due to internal pressure?

I think it is my turn to say you are getting very confused
Henry.

It is a fact that most 'cepheids' appear to have a companion...

It is a fact that something around half of _all_ stars
are in binary systems so there is no reason why Cepheids
should be an exception.

which means they
are in some kind of orbit.
I reckon the movement of their surfaces would feature similar radial
velocities
to those of an orbit. It is distinctly possible that the huffing is linked
to
the orbit period. It is also possible that the stars are in tidal lock ..

It is certainly possible, especially for close binaries,
but less likely for those with greater separations.

and
distorted into some kind of dumbell shape,

No, each would be more like an egg shape. Look up "Roche Lobe".

leading to a brightness variation as
they orbit....but that wouldn't account for the short periods of many of
them.

It wouldn't account for any where the period of the Cepheid
differs from the orbital period, nor does it account for those
that are not in binary systems.

George