In Romeo and Juliet, what happens to Romeo's old love?

Since
Rosaline, the girl thatis infatuated with at the opening of the play, is never an actual
character in the play, there is no mention of what happens to her.

She is
mentioned on the list of persons invited to the Capulet's feast, and this is the device that
Shakespeare uses to contrive to have Romeo be in a position to actually meet and fall in love
with .  Rosaline is Capulet's niece, so not only does her existence as Romeo's first love
provide the opportunity (Capulet's feast) for Romeo to meet Juliet, she also provides the
precedent for him to fall in love with a girl from the House of his family's enemy.

The existence of Rosaline also provides the circumstance, in the opening Act of the
play, for the audience to get a good picture of what kind of lover Romeo is.  As he moons and
mopes about the stage, they see how dramatic and poetical he is about love.  These
characteristics will present a nice foil to the more logical and practical minded lover that
Juliet turns out to be.  This is an ironic twist to the classic "boy/girl" lovers'
character traits -- the young man who seems almost "girlie" in his approach to love
and the young woman who is very logical, almost masculine in hers.

Theenters
before Act Two begins and says:

Now old affection
[Rosaline] in his deathbed lie

And young
affection [Juliet] gapes to be his heir

That
fair for which loved groan'd for and would die,

With tender Juliet match'd is
now not fair.

And from this moment the character of
Rosaline is, in essence, "killed off."  The play has no further use for her.  And, as
she is a fictitious character in a play, we, the audience, have no idea what "happens"
to her.