*The context for this question is the theory ZFC + a measurable cardinal, although answers not in this context would also be interesting to me.*


In a project I'm working on, the following class of reals has emerged, and I'd like to understand it better:


Say that a real $r$ is **hyperarithmetic modulo ordinals** (and write $r\in\Delta\_1^{1, ord}$) if $r$ can be defined in a $\Delta^1\_1$ way relative to some ordinal parameters. Formally, $r$ is $\Delta\_1^{1, ord}$ if there is a tuple $\overline{\alpha}$ of ordinals and a pair of formulas $\varphi(x, \overline{y}), \psi(x, \overline{y})\in\Sigma^1\_1$ in only the displayed parameters such that, whenever $\overline{c}$ is a tuple of reals coding copies of $\overline{\alpha}$, we have $$r=\{n: \varphi(n, \overline{c})\}=\{n: \neg\psi(n, \overline{c})\}.$$


Now, this class of reals is *much* bigger than $\Delta^1\_1$. For example, Kleene's $\mathcal{O}$ is $\Delta\_1^{1, ord}$: $\Phi\_e$ is well-founded iff it embeds into $\omega\_1^{CK}$. 


EDIT: as soon as I posted this, I realized that this can be pushed further: unless I'm missing something, every constructible real is $\Delta\_1^{1, ord}$. So the right question now is:



> 
> Is $\Delta\_1^{1, ord}=L\cap\mathbb{R}$? 
> 
> 
> 


Currently I suspect the answer is "yes" - note that $\Delta\_1^{1, ord}$ is forcing absolute! - but I don't see how to prove it. 


A more conservative question is:



> 
> Is there a $\Delta^1\_3$ real which is not $\Delta^{1, ord}\_1$?
> 
> 
> 


In general, any information about this class (and its obvious variations - e.g. $\Sigma^{1, ord}\_n$) would be valuable to me. I suspect this is all very well-known in the descriptive set-theory community, so I've added the "reference-request" tag.



Which reals are "hyperarithmetic modulo ordinals"?
In fact $\Delta^{1, ord}\_1 = \mathbb{L} \cap \mathbb{R}$.


For suppose $(\phi(x, \overline{y})$, $\psi(x, \overline{y})$, $\overline{\alpha})$ is as you describe, defining $r \subset \omega$. We show $r \in \mathbb{L}$. For convenience we suppose $\overline{\alpha} = \alpha$ is a single ordinal; this is no loss, by coding.


Now if $\alpha$ is countable in $\mathbb{L}$ we are done by Shoenfield. If $\alpha$ is not countable in $\mathbb{L}$ we proceed as follows.


Let $\mathbb{P}$ be the usual forcing notion for adding a surjection $f: \omega \to \alpha$. Let $G$ be $\mathbb{P}$-generic over $\mathbb{V}$; then $G$ is also $\mathbb{P}$-generic over $\mathbb{L}$ so we can consider $\mathbb{L}[G] \subset \mathbb{V}[G]$. Now, by Shoenfield, we have that in $\mathbb{V}[G]$, whenever $c$ codes $\alpha$, then $\{n: \phi(n, c)\} = r$. By Shoenfield again, this remains true in $\mathbb{L}[G]$.


Henceforth we work in $\mathbb{L}$. Since $G$ could have been chosen to contain any given $p \in \mathbb{P}$, we have shown that for each $n$, $\mathbb{P}$ forces that $\phi(n, \dot{c})$ holds iff $n \in r$ (where $\dot{c}$ is a generic code for $\alpha$). Hence $r$ is definable in $\mathbb{L}$.


