"Addressing my party invitation to a lesbian couple"

How do I address my party invitations to a lesbian friend and her partner, when I don’t know her partner’s name?

Unless your friend goes through partners so fast that today’s lover might well be tomorrow’s ex, shoot a quick e-mail to your friend saying something like “I don’t seem to have your partner’s full name in my electronic address book. Could you please send it to me?” Alternatively, send the invitation to your friend, using her name and the words “and partner” in the address line. In that case, if your friend is a good guest, she’ll reply along the lines of “I’d love to come and bring Dani, too. Just so you have it, her full name is Dani Shapiro. Thanks for asking us both.”