Orientable double covers for non-orientable manifolds If I have two non-orientable connected manifolds such that their orientable double covers are homeomorphic, can anything be said about the manifolds? Are they homeomorphic? 
 A: They are in general not homeomorphic. Here is a counterexample.
Consider the manifold $X = S^2 \times S^2 \times S^2$. We define two free $C_2$ actions on X, where $C_2$ is the cyclic group of order $2$. 
The first is $(x,y,z) \sim (-x,y,z)$ and the second is $(x,y,z) \sim (-x,-y,-z)$. You obtain two quotients with are on the one hand $\mathbb{R}P^2\times S^2 \times S^2$ and on the other hand some $6$-manifold $M$.
Since both actions are orientation reversing you see that both quotients are non-orientable. Moreover since $X$ is simply-connected both quotients have $C_2$ as fundamental group. This directly implies that $X$ must be in both cases the orientation double cover (since there is only one connected 2-sheated covering over the quotients).
Now we claim that $M$ and $\mathbb{R}P^2\times S^2 \times S^2$ are not homeomorphic. This you can prove using rational cohomology. For $\mathbb{R}P^2\times S^2\times S^2$ you can do this by Künneth, and for $M$ I at the moment do not know of a better reason than using the Cartan-Leray spectral sequence to show that $H^*(M;\mathbb{Q}) \cong H^*(X;\mathbb{Q})^{C_2}$ where the last means the $C_2$-invariants of the cohomology of $X$ by the induced $C_2$-action. Now you can distinguish the two manifolds by H^2 and H^4.
It would be interesting to explicitly identify the manifold $M$ if this is possible.
A: Here's a slightly easier example than mland's.
Let $X = S^2\times S^4$  Then $X$ is the universal (orientation) cover of both $\mathbb{R}P^2\times S^4$ and $S^2\times \mathbb{R}P^4$.  By the Kunneth formula, $H_2(\mathbb{R}P^2\times S^4) \cong 0$ while $H_2(S^2\times \mathbb{R}P^4) = \mathbb{Z}$.
Somewhat relatedly, (if you want a lower dimensional example), $T^2\times S^2$ covers both $K\times S^2$ and $T^2\times \mathbb{R}P^2$ where $K$ denotes the Klein bottle.  In this case, the fundamental group of the first is nonabelian while the fundamental group of the second is abelian.
