A/N: So here's all my OC nonsense. Basic rundown: Solis is a renegade Time Lord in their twelfth regeneration, who fought in the Time War and deserted before it could end. After some time travelling alone they met Cassie, an astrophysics prof, on Earth, and she became their companion and later queerplatonic partner. There's a bit more about them on my Tumblr, but most just exists in my head, so if you by chance ever get curious feel free to ask me about them lmao.

"All I'm saying," muttered Solis, "is that you lot are going about it all wrong. A standard engine can only get so fast, but–"

"Dematerialization is the future," finished Cassie, a long-suffering smile on her face. "I know, Sol, I've been."

She didn't need to move her book out of the way to know they were pouting up at her, giving her that doe-eyed look of which no being of their age and hauteur should be capable. They were laid out across the length of the sofa, their head in her lap and their feet propped against the far armrest, ankles crossed in thinly-veiled impatience. They'd been waiting nicely for her to finish her reading on twenty-third century relativity but, despite the fact that they'd been busy nonstop for days, they were very eager to take her out sightseeing. Or at least they had been; now, they seemed to be rather preoccupied.

"Oh, you must remind me to take you to the twenty-five hundreds," they said, "there's this lovely Earth spacecraft that–" They broke off in a yawn, a shiver running visibly down their body. Cassie peered around her book, amused, to see them rubbing at their eye, trying to shake off their obvious tiredness.

"Why don't you just go to bed?" she suggested. "We can go out later."

Solis shook their head. "I'm not tired," they protested. "As I was saying–"

"Sol," she said, drawing it out pleadingly. "C'mon. It's been forever."

"It's been four days," they corrected.

She gave them a pointed look. They huffed, returning her gaze defiantly. Then they yawned again, and their expression turned sheepish.

"Well," they grumbled, "perhaps we could go a bit later."

Cassie smiled down at them, and freed one hand to ruffle through their hair. "Go get some sleep," she ordered softly. "Your fancy engines will wait."

Solis hesitated then, glancing towards the door of the library. A slight shadow crossed their face; they didn't like to sleep alone, she knew, and they weren't particularly good at hiding it. She waited for the inevitable.

"Could I… stay?" they asked, far too tentative given the fact that this was a regular routine for them.

"Yeah, course," she said, watching fondly as gratitude flickered in their eyes. She set down her book for a moment, stretched and grabbed the blanket folded on the back of the sofa, helping to arrange it over their prone form. They wrapped it tight around their shoulders, turning on their side to curl up with their legs drawn up to their chest. A contented little smile played across their lips.

Cassie held her book open against the armrest and reached down with her free hand, draping her arm over Solis' shoulder; they reached up to take her hand in theirs, resting it against their chest. They'd already closed their eyes.

"'Night, Sol," she murmured, rubbing her thumb over the back of their hand.

"Could've gone now," they mumbled.

"I know," said Cassie, humouring them.

"Don't need–" they yawned– "sleep."

"I know." She smiled. "Goodnight, Sol."

They sighed, submitting, and burrowed just a bit further under the blanket. "'Night."