What are the indecomposable classes on a del-Pezzo surface? Let $X_k$ be $\mathbb{P}^2$ blown up at $k$ points (where $k$ is $0$ to $8$). 
Let $\beta \in H_2(X_k, \mathbb{Z}) $ be a homology class given by 
$$ \beta := n L + m_1 E_1 + \ldots + m_k E_k $$ 
where $L$ is the homology class of a line and $E_i$ are the exceptional 
divisors. My question is as follows: 
Which homology classes are $\textit{indecomposable}$? By definition, a homology 
class is indecomposable if: 
a) It can be represented by a non constant holomorphic map 
$u:\mathbb{P}^1 \longrightarrow X_k $  and 
b) It cannot be written as $\beta = \beta_1 + \ldots \beta_n$ for some $n \geq 2$ 
such that each $\beta_i$ has a non constant holomorphic representative 
(as a map from $\mathbb{P}^1 $ to $X_k$). 
My motivation for asking the question is as follows: I am explicitly trying to 
work out what is $N_{\beta}$, the number of rational curves in $X_k$ (through the right number of generic points) that represent the class $\beta$. Kontsevich and Mannin have given a recursive formula for this number in their paper (page 29) 
http://www.ihes.fr/~maxim/TEXTS/WithManinCohFT.pdf 
In order to actually calculate what is $N_{\beta}$, we need some initial conditions. I think the initial condition is that $N_{\beta} =1$ if $\beta$ is indecomposable.      
$\textbf{Added Later:}$ Based on Mark's observation (and one further question I have about Kontsevich Mannin's paper) I have posted a separate question on mathoverflow 
Are genus zero Gromov Witten Invariants on Del-Pezzo surfaces enumerative? 
 A: Let me just summarize the comment thread in case someone more knowledgeable is willing to intervene.
Artie observes that The Cox Ring of a Del Pezzo surface by Batyrev--Popov shows that every effective class on a del Pezzo with $k \geq 2$ is a sum of $(-1)$-curve classes, with one exception.  This means that no effective classes can be indecomposable in the sense above except for those of $(-1)$-curves.  All the $(-1)$-curve classes are clearly indecomposable and have $N_\delta = 1$.  And of course it is easy to write down these classes explicitly.
The one exception is that if $k = 8$ the anticanonical class is not a sum of two nonzero effective classes.  This class is represented by (the strict transforms of) the pencil of cubics through the 8 blown up points.  There are 12 singular cubics in the pencil, hence 12 that are rational, and it seems that we should have $N_\delta = 12$ by the OP's definition.  The catch is that Kontsevich-Manin "expect" $N_\delta = 1$ for an indecomposable class on a del Pezzo (page 29 of the article in the question, following Claim 5.2.3).
So the question is, have we misunderstood something along the way (more likely, e.g. the definition of $N_\delta$), or did the authors forget a minor case?  (A secondary question: is this already worked out somewhere?)
