how to use combineLatest/zip but only react to emits of second observable and ignore emits from first observable

So I have this property initialization:

this.currentMapObject$ = zip(this.mapObjects$, this.currentMapObjectsIndex$,
      (mapObjects, index) => mapObjects[index]);

I only want to emit this.currentMapObject$ if this.currentMapObjectsIndex$ emits but not if this.mapObjects$ emits.

As far as I know now this.currentMapObject$ will emit even if a single property of an item of this.mapObjects$ changes. I want to ignore all changes to this.mapObjects$ and only take its current value, but I DO want to react to this.currentMapObjectsIndex$ changes