What produces gravitational waves with "periods between about 100 - 8000 seconds"?
Any binary system produces gravitational waves at twice it's orbital frequency, i.e. with periods of half it's orbital period. So binary systems with periods between 200s and 16000s will produce such waves.
We can use Kepler's third law to say something about these:
$$ a = \left(\frac{GM}{4\pi}\right)^{1/3} P^{2/3},$$
where $P$ is the orbital period, $M$ is the total mass of the binary system and $a$ is the orbital separation.
For a binary with $M\sim 1M_{\odot}$ and $200&lt;P&lt;16000$s, then $0.11 &lt; a &lt; 2.00\ R_{\odot}$. Since normal stars of mass $\sim 0.5M_{\odot}$ have radii that are similar to this, then the stars would probably need to be stellar remnants (white dwarfs, neutron stars or black holes) except right at the longest period end, where it might be possible to observe W Uma binaries. More massive binaries have separations that increase as $M^{1/3}$, but the radii of normal stars increases more like $M$, so this conclusion is even firmer at larger masses.
It could be possible to have a compact binary involving a low mass star plus a compact object - perhaps a Roche lobe filling one, so as well as &quot;double degenerates&quot;, the long period end of this range would include Cataclysmic Variables and Low Mass X-ray binary counterparts, with orbital periods of a few hours. Here is a prime example Time domain astronomy and fastest eclipsing binary ZTF J1539+5027 (+20 mag, 6.91 minutes): How to measure its minimum brightness?
Of course gravitational wave strain goes as something like $M P^{-4/3} d^{-1}$, where $d$ is the distance. These binaries are much longer period than the (presumably rare) massive, merging black holes seen so far and so probably need to be close, in our own Galaxy, to be detected.
e.g. LIGO was capable of detecting $M \sim 30 M_{\odot}$ merging black holes, with $P \sim 0.02$ s at distances of a billion light years. A similar strain amplitude would be produced by a $M\sim 2M_{\odot}$ binary with $P= 200$ s at a distance of 300 light years.