He hardly ever mentions them. The few times he does are so brief that she almost misses it. Usually it's by accident - he'll just be rambling on about some place he wants to show her or something he wants to do with her, and then a name will slip out. Like how they should tour the planet Midnight together, because last time Donna was too busy tanning to come with him, or that he wants to show her Apalapucia, but it will be a lot safer than last time with the Ponds. Sometimes he'll momentarily freeze, and look right through her with a sad, nostalgic look in his eyes. But he recovers so quickly, continuing his rambling while dancing around the console, that half the time she wonders if she imagined it. She tries her best not to bring up the topic much, mostly because she can't bear to see his eyes. It's those times that she realizes how old and haunted they are, how deep down he's just a sad old man with a box. She wonders what will happen when she eventually exits the Doctor's life for good (not that she'd ever want to, but she knows it will happen one day). She wonders what will happen when the Doctor gets a new companion to fly amongst the stars with. She wonders if she will become nothing more than a new shadow in the Doctor's eyes, a new person to shove in the corner of his mind. A new ghost to follow him wherever he goes.