Zeta: Yet another dc-dc topology
It is a bit like sepic and Cuk, in that the output can be higher or lower than the input.
And that there are two inductors and a series capacitor.
And it is a bit like sepic in that the output polarity is the same as the input polarity. (Cuk swaps polarity).
And, it is also called inverse-sepic.
Pick your favourite chip company
Texas Instruments has a nice description of zeta in this application note.
Linear Tech also offers zeta chapter and verse.
As does Microchip, and in this one is how to implement a battery charger with the topology on a microcontroller.
Analog Devices has one too.
It says if sepic is considered a modified boost converter, then zeta is a modified buck converter.
Zeta has low output ripple and high input ripple (opposite to sepic. Cuk has both low).