There is no good legal reason for either Joe or Hunter Biden to testify at the Senate impeachment trial. But there are legitimate reasons for the Senate later to hold ordinary oversight hearings about the Bidens’ dealings in Ukraine.

The Bidens should be asked to testify under oath, under penalty of perjury charges, but they should be given “use immunity” for the substance of anything to which they testify.

I argued yesterday that their testimony is irrelevant to the Senate trial of President Trump. The short version: Even if the Bidens did act corruptly, the case against Trump is that he had no authority nudging a foreign state to go after them — much less in the way he went about it, using U.S. aid as leverage.

To that understanding, I should add that it’s absurd to say Hunter Biden was ever at risk in the “investigation” into the Burisma energy company’s former executive, for the simple reason that Biden joined Burisma’s board after the actions that had prompted the investigation. (Most of the investigation involved events from way back in 2002, which was 12 years before Biden joined the board.)

So, then, why should the Senate hold hearings on the Biden’s actions? Simple: That which is irrelevant to an impeachment trial can still be, broadly speaking, unethical, even if it isn't illegal. It may also have relevance to U.S. policy. For example: If Hunter Biden’s (probably legal but sleazy) profiteering on Burisma’s board helped create alliances within Ukraine that affected the attitudes of various Ukrainian power brokers vis-a-vis Trump and, eventually, the administration as a whole, then our diplomats need to unravel it all in order to ameliorate the problem.

With that in mind, here are the sorts of questions senators may want to ask the Bidens.

To Hunter:

How did you come to be a board member of Burisma? Who first approached you? What was the reason given as to why you were wanted on the board? Did you or your partner Devon Archer consult with your father before you accepted the position? Did you consult with U.S. government ethics officials? If so, what did your father or the ethics officials tell you? If not, then what was the subject of Archer’s meeting with your father two days before you joined Burisma’s board?

Did it concern you at all that you were possibly being used for unethical purposes? Did it concern you that your father was the point man for the whole U.S. government for policy toward a country, riven by corruption, in which the business you were joining was largely based — especially since that company’s head honcho had been accused of corruption? Did you not worry that it might cause an apparent conflict of interest for your father? Did you not worry this could hinder the effective pursuit of U.S. interests? Did you even care?

Once you were there, and without divulging any proprietary information of the company, what were your duties on the board? How many hours per week would you say you spent doing actual work?

Most importantly: In your work for Burisma, did you see evidence of endemic corruption in the Ukrainian financial or political system? If so, was it of a nature that could detrimentally affect U.S. interests, diplomacy, or the efficacy of financial or military assistance to Ukraine?

To Joe:

Did you know that Hunter was joining Burisma’s board before he did so? Did you advise him on whether he should join? Did it raise any concerns about the reality, or even the appearance, that he was profiteering from his association with you, especially since you had been publicly identified as the Obama administration’s point man on Ukrainian policy? Knowing that Burisma’s executive had been under investigation, did you have any concerns that a conflict of interest might arise between U.S. diplomatic interests and your son’s role with Burisma? Did you seek an official government ethics advisory about whether your policy oversight and your son’s board position created a conflict of interest? If so, what exactly was the wording of your inquiry, and what exactly was the answer you received?

Why did you not tell President Barack Obama you would recuse yourself from Ukrainian policy because of Hunter’s position? Did you even consider doing so, even after numerous American media outlets criticized the appearance of a conflict?

To your knowledge, were you ever approached by anyone acting on behalf of Burisma or any of its contractors, subsidiaries, affiliates, consultants, lobbyists, or nonprofit organizations to which it had donated money?

In all your dealings with Ukraine, did you encounter any signs that any Ukrainians acted differently toward you or the United States because of your son’s lucrative board position?

Ukraine is indeed a key strategic partner of the United States. The last year’s events have made it one fraught with dangerous cross-currents. The Senate should question the Bidens under oath so we can better understand those currents, and better deal with them moving forward.