The 21st century consulting detective Sherlock Holmes and the time-traveling alien from Gallifrey known as the Doctor might live in the same universe after all. A new set picture from the fourth season of Sherlock contains a hidden shout-out to a deep-cut from Doctor Who lore.

In a new photograph of the interior of 221B Baker Street, one particular book on the shelves next to John (Martin Freeman) and Sherlock (Benedict Cumberbatch) was written by a “Lavinia Smith.” In analyzing the photo, The Radio Times pointed out that Lavinia Smith is the aunt of Sarah Jane Smith from Doctor Who. In Who continuity, Lavinia was only mentioned in the Third Doctor episode “The Time Warrior.” But, she appeared in the flesh in the one and only episode of K-9 and Company; a 1981 Doctor Who spin-off that never became an ongoing series.

In Doctor Who lore, Lavinia Smith (Mary Wimbush) raised Sarah Jane Smith after Sarah Jane’s parents passed away. Sarah Jane Smith, meanwhile, is arguably the most famous of all Doctor Who companions. Played by Elisabeth Sladen, Sarah Jane was an investigative reporter and also first appeared with Jon Pertwee’s Third Doctor in “The Time Warrior” in 1973, but continued as a series regular with Tom Baker’s Fourth Doctor all the way through 1976.

The Smiths: Lavinia and Sarah Jane
The Smiths: Lavinia and Sarah Jane

Subsequently, Sarah Jane also reappeared in “The Five Doctors” in 1983, and then again in the 2006 episode “School Reunion.” After her reintroduction into the Whoniverse, Sarah Jane finally got her own spin-off called The Sarah Jane Adventures in 2007. The show lasted for three seasons and was canceled after Sladen’s death in 2011.

Why would Lavinia Smith be connected with Sherlock Holmes? From a narrative standpoint, it actually makes a little bit of sense: Lavinia Smith was a renowned virologist — someone who studies microscopic parasites — which is 100% something Sherlock would be into.

Steven Moffat with Benedict Cumberbatch and Matt Smith
Steven Moffat with Benedict Cumberbatch and Matt Smith

While this connective thread is pretty thin, as of this writing, Steven Moffat is still the showrunner for both Doctor Who and Sherlock. If Lavinia Smith exists in both universes, then we can continue to hold out hope that the Doctor himself will take Sherlock and John for a ride in the TARDIS. How else is Sherlock going to visit that alternate Victorian version of himself?

Sherlock returns to the BBC in 2017.

Doctor Who returns to BBC and BBC America on Christmas Day 2016.

Photos via BBC/Inverse