system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Are the Tau Necrontyr

They both have no warp signature, they both have the same skull shape,  they both have a thing for magnetic weapons and hover tanks, and both have short life spans.
gpt
First off, neither the Necrontyr nor the Tau lack a warp signature. They're just extremely weak. If Humans are a torch, Tau are a match, and Eldar are a bonfire. 

Second, the Tau and Necrontyr seem to have some significantly different anatomies, despite similar heads. If the Necrons are representative of Necrontyr bodies, then their feet and leg structure is different, they have more digits on their hands, more narrow rib structure, and generally different posture. 

Third, the Tau have short biological lifespans, but don't seem to be plagued by cancers and other degenerative diseases, which were the bane of the Necrontyr. 

Fourth, the Eldar also have a thing for magnetic weapons and flying vehicles. In fact, Tau tech is more in line with lower-grade Eldar than with the techno-sorcery of the Necrons.

So, taken as a whole, it's unlikely that they are the same species.

BUT. And there's a big but. SOMEONE does seem to have been helping the Tau. Whether it's the Necrons, the Old Ones, or some unknown other player, remains unclear, but there have been several odd occurrences throughout Tau history that are just way too convenient to ignore.