Marcus Caelius Rufus    (the Social Set of Catullus)
Had an affair with Clodia some time after her husband had died in 59 B.C.
Replaced Catullus as her lover, perhaps while Catullus was off in Bithynia.
Successfully defended by Cicero on a charge of attempting to poison Clodia in 56 B.C. (Pro Caelio Speech)
Praetor in 48 B.C. (under Julius Caesar, but removed from office and killed in riots.
Poems addressed to "Caelius" (but Poem 100 may refer to a different Caelius):
Poem 58CAELI, Lesbia nostra, Lesbia illa
Poem 100CAELIVS Aufillenum et Quintius Aufillenam
Poems addressed to "Rufus" (this may be a different Rufus, but Poem 77 obviously fits the situation of Catullus, Caelius, and Clodia):
Poem 69NOLI admirari, quare tibi femina nulla
Poem 77RVFE mihi frustra ac nequiquam credite amice