Can't set a project series' owner

Note that I am able to set the series' owner using launchpadlib:

from launchpadlib.launchpad import Launchpad, EDGE_SERVICE_ROOT
launchpad = Launchpad.get_token_and_login('just testing', EDGE_SERVICE_ROOT, cachedir)
elisa = launchpad.projects['elisa']
onezero = elisa.series[2]
onezero.owner = launchpad.me
onezero.lp_save()

The project series owner is really the registrant. The project owner is the effective owner regardless of who's name in in the series field. The series owner has no power.

When a user is set as the Release Manager of a series he is given the same access privileges as the project owner. A project driver can creates a series, and he is automatically made the release manager, ensure that the driver has all the power to start, manage, and complete a release.

So the series owner *is* deprecated and should not be used for anything, but series.maintainer should be used instead, right?

I am just asking so I can file a proper bug in rosetta to fix this there.

And I am wondering why that field still exists (and gets populated)...

No. owner is the registrant. There is no maintainer and there should not be given that is is ephemeral. The release managers (series.driver) is the only role besides the owner on the object, it is exists so that the project owner can delegate responsibly to plan a release.

It does not seem semantically correct to me that the owner be the same entity as the registrant.
What if Alice registers a series for project foo, then decides to withdraw from the project and doesn't want to be involved any longer (and that includes not receiving any mails related to activity on whatever items she registered)? Prior to withdrawing, she should be able to transfer the series' ownership to Bob, while remaining the series' registrant.

It is not semantically correct. The owner is the registrant, thus has no power. The role you are concerned about is the *Release manager*. Alice can make someone else the release manager.

OK, that makes sense. The release manager is what's called "driver" in launchpadlib, right?
I guess my original concern calls for a bug in Rosetta, that should inform the series' driver when some translation templates were imported, not the owner.

Just had a look at the code, and the _uploader_person_pot method first tries the series driver, before falling back to the series owner. The behaviour I observed was due to the fact that no driver was defined for the series, which I fixed.
Not a bug it is then.

I See. Yes the series.driver is a special driver, he is also called the release manager and has more privileges. We had requests to create a release manager role, which overlapped with the driver role. Many developers did not know that the driver role was like a release manager. We changes the UI a few months ago to use the term Release manager which is a term more common in software development. We also gave the person/team in that direct role the power to do what he is expected to do--plan and manage the entire series. See http://blog.launchpad.net/general/more-power-to-the-release-manager

One more thing: as mentioned in comment #1, changing a series' owner (a.k.a. registrant) is possible using launchpadlib. Is this expected, or is it a bug?

It is expected. This is bug 207532 which affect several objects.