Hibiscus syriacus = Rose of Sharon
A couple of named cultivars are Diana and White Chiffon.

Hibiscus moscheutos = Hardy Hibiscus
"Luna White" is a named cultivar.

There are a number of species in the genus Hibiscus. If you could post a picture here of your plant, I'm sure someone will be able to give you an ID.

Rosie

Here is a link that might be useful: Google results for

Marilyn,
Best bet can be the leaf pattern. Hib. Syriacus has leaves that look like the link below. If they do not look like this, you have eliminated one possibility.
Ed

Here is a link that might be useful: Rose of Sharon Leaves

Marilyn,
Best bet can be the leaf pattern. Hib. Syriacus has leaves that look like the link below. If they do not look like this, you have eliminated one possibility.
Ed

Here is a link that might be useful: Rose of Sharon Leaves

This is the hibiscus I was wondering about.

Marilyn,
This appears to be a very early hybrid called "White Wings" branching is not as compact as newer CV's, but it has many great attributes. The house in California my mom inherited when my Grandmother passed has a many decades old plant. It is exceeding 15 foot high at this time. I have a link below to the image my mother took last fall. Scroll down a bit to see this monster.
Ed

Here is a link that might be useful: White Wings Link

Thanks Ed for the site. There are a lot of beautiful colors of hibiscus.